In the bowl of a stand mixer fitted with the paddle attachment, beat butter at medium speed until smooth, 1 to 2 minutes. Add sugars, ground coffee, salt, and orange zest; beat at medium-low speed until smooth, about 1 minute. Add egg yolk and vanilla. Beat until combined, about 1 minute. With mixer on low speed, add flour to butter mixture in two additions, beating until combined. Shape dough into a 12-inch log; wrap in plastic wrap, and refrigerate until firm, at least 2 hours.: You will notice the butter soften and become pale as you beat it, releasing a nutty aroma and creating a creamy base that traps air, which leads to a lighter texture. Stop when the surface looks even and satin like, scraping down the sides to ensure uniformity. If you underbeat, the sugars will not incorporate properly and you may end up with an uneven crumb. If the butter becomes oily, it has been overworked or warmed too much, and the dough will spread during baking.
Preheat oven to 325°. Line 2 rimmed baking sheets with parchment paper.: As the confectioners’ sugar and granulated sugar fold in, you should smell the citrus oils lift and the faint coffee aroma mingle with the butter. Mixing at medium low prevents the dry ingredients from puffing into the air while still ensuring even distribution. A common mistake is adding the dry ingredients too quickly, which can create pockets of sugar or coffee. If you see gritty spots, pause and scrape the bowl to finish blending.
Using a serrated knife, cut dough crosswise into 1/4-inch-thick rounds. Place at least 1 inch apart on prepared pans.: The egg yolk will add silkiness and body, while the vanilla rounds the flavors. You should see the mixture become slightly smoother and glossier. Overbeating now can incorporate too much air, which may alter the final texture. If the mixture looks curdled, a brief low speed beat and a scrape will usually bring it back together.
Bake until tops are set and bottoms are lightly golden, 15 to 18 minutes. Let cool on pans for 1 minute. Remove from pans, and let cool completely on a wire rack.: As you add the sifted Swans Down Cake Flour , the dough will move from glossy to a soft, pliable mass. Adding the flour in two portions prevents overmixing and helps you stop mixing as soon as the streaks disappear. You want a cohesive dough that still looks tender, not a dense ball. Overmixing develops gluten and makes the cookies tough, so stop when the last streak of flour vanishes.
In a small microwave-safe bowl, heat chocolate on high in 10-second intervals, stirring between each, until melted and smooth (1 to 1 1/2 minutes total). Stir in shortening until smooth and well combined. Spoon chocolate mixture into a small pastry bag or resealable plastic bag; cut a 1/4-inch opening in tip or corner. Drizzle chocolate mixture over shortbread as desired. Let stand until set before serving, 15 to 20 minutes.: When shaping, press gently so the log is compact and even in diameter, which helps yield uniform slices. The dough should feel cool and slightly stiff when ready to slice. Chilling firms the butter so the cookies retain shape during baking, and it also allows flavors to meld. If you skip chilling, slices may spread and lose their pretty edges.
Preheat oven to 325°: Preheating ensures the oven reaches the right steady temperature so the cookies bake evenly, developing set tops and lightly golden bottoms. I always position racks in the center and allow the oven to fully stabilize before placing pans inside. A common error is placing cookies in a not fully preheated oven, which causes unpredictable spread and texture; wait until the temperature is reached for best results.
Line 2 rimmed baking sheets with parchment paper: Using parchment shields the bottoms from direct heat and helps the cookies bake evenly with gentle browning. It also makes cleanup effortless and prevents sticking. If you use an unlined pan, you risk uneven coloring and potential sticking, so I suggest parchment for consistent outcomes.
Using a serrated knife, cut dough crosswise into 1/4-inch-thick rounds. Place at least 1 inch apart on prepared pans: The serrated knife gives a clean slice through the chilled log, producing neat rounds with minimal crumbling. Arrange them with space so hot air can circulate and the edges crisp. If the slices crumble, return the log to chill briefly; trying to press them back together often leads to misshapen cookies.
Bake until tops are set and bottoms are lightly golden, 15 to 18 minutes. Let cool on pans for 1 minute. Remove from pans, and let cool completely on a wire rack: As they bake you will see the edges take on a faint gold while the centers remain pale, and the kitchen will be scented with citrus and butter. Letting them rest on the pan briefly stabilizes the structure before transferring to a wire rack for complete cooling, which prevents sogginess. Pulling them too early can cause breakage, and leaving them too long on hot pans can overbrown the bottoms.
In a small microwave-safe bowl, heat chocolate on high in 10-second intervals, stirring between each, until melted and smooth (1 to 1 1/2 minutes total): The chocolate should melt into a glossy pool as you stir, releasing a warm chocolate aroma that pairs with the coffee notes. Short bursts of heat and frequent stirring keep the chocolate smooth and prevent scorching. A frequent pitfall is overheating, which will cause the chocolate to seize into a grainy mass; if that happens, adding a small amount of warm fat can sometimes rescue it.
Stir in shortening until smooth and well combined: The small amount of all-vegetable shortening creates a silky emulsion with the melted semisweet chocolate , giving it a glossy sheen and a stable texture that sets neatly on the cookies. Stir until the mixture looks homogeneous and satiny. Adding too much shortening will soften the set finish, while too little may leave the drizzle dull or slightly stiff.
Spoon chocolate mixture into a small pastry bag or resealable plastic bag; cut a 1/4-inch opening in tip or corner. Drizzle chocolate mixture over shortbread as desired: The drizzle should fall in thin ribbons that create pretty patterns and a shiny finish. Work over a sheet to catch excess and practice a few strokes to control flow. If the chocolate is too warm it will be runny and hard to control, if too cool it will clump and not drizzle neatly. Rewarm gently if needed, in very short bursts.
Let stand until set before serving, 15 to 20 minutes: As the chocolate cools it will firm to a pleasant snap and the smells will mellow into a welcoming chocolate citrus scent. Give them time to set fully so the drizzle does not smear when handled. A mistake is rushing service, which results in fingerprints and smudged decoration. If you need them sooner, a brief chill will speed the setting.