Preheat oven to 350°F. Line two cookie sheets with parchment paper or silicone baking mats.: You will notice the kitchen warming as the oven nears temperature, and the air will carry a faint toasty hint even before the cookies go in. Proper preheating ensures even rise and predictable browning, which is why I always wait for the oven to reach 350°F before starting. Use parchment or silicone so bottoms do not color too quickly, and the cookies slide off easily. A common mistake is to put the tray in too soon, which can throw off baking timing and produce unevenly baked cookies.
Beat butter and oil with a hand or a stand mixer, then slowly mix in powdered sugar, vanilla, and salt. Slowly add flour and mix until a thick dough forms. This will take a while with a hand mixer!: As you start mixing, the blend will go from shiny liquid to a pale, creamy mass, and you can hear the mixer change pitch as the texture thickens. This creaming step integrates fat and sugar, creating the tender crumb you want. Add the powdered sugar gradually to avoid a dusty cloud and to help it dissolve fully into the fat. Avoid overbeating at high speed, which can aerate the dough too much and change the texture of the finished cookie.
Scoop 1 or 2 tablespoon sized balls of cookie dough onto cookie sheets. Flatten slightly with the palm of your hand. Bake until the edges just turn brown and tops are just no longer translucent (2 tablespoon sized cookies: 10-14 minutes, 1 tablespoon sized cookies: 8-12 minutes). Cool completely before removing from cookie sheets. Store in an airtight container for up to 3 days or freeze for up to one month.: The moment you add the all purpose flour , the mixture will evolve from glossy and loose to a cohesive, dense dough that holds together when pressed. Work in the flour gradually and stop mixing as soon as the dough comes together; overmixing develops too much gluten and makes the cookies tougher. If the dough feels crumbly, give it a few gentle turns with a spatula until it binds, but avoid heavy kneading which can create a dry, stiff result.
For M&M Shortbread Cookies: add M&Ms on top of the cookie dough ball before baking. For 2 tablespoon sized cookies, I suggest using 5-6 plain M&Ms, any color.: When you scoop the dough, it should feel supple and slightly cool, and rolling or flattening with your palm gives uniform thickness for even baking. Flattening slightly helps the center bake through while maintaining a tender interior. Placing dough balls too close together leads to merging during baking, so space them sufficiently apart to keep round shapes.
For Chocolate Dipped Shortbread Cookies: melt 4 ounces of white or semi-sweet baking chocolate according to package directions. Dip cooled cookies halfway into chocolate, tapping off the excess, and then place on a wax paper lined cookie sheet and chill until hardened. Sprinkle with crushed candy canes, toffee bits, nuts, or sprinkles while still wet, if desired.: During baking you will see the tops change from glossy to matte and the edges will take on a very light golden hue. That subtle browning is the cue to pull them from the oven, keeping the centers tender. If you wait for deep color you will lose the soft chew, so watch closely in the final minutes. A typical timing guide varies by size, and overbaking is the most common mistake leading to dry cookies, so err on the shorter time and allow carryover heat to finish the center.
For Thumbprint Cookies: make 1 tablespoon sized balls of cookie dough. Roll them between your hands to form a ball. Use the back of a wooden spoon to slightly make an indent in the center of each cookie. Bake until lightly golden on the bottom (8-12 minutes) and then re-press hole with the wooden spoon handle as needed. Cool completely. For filling, fill with about 1/2 teaspoon of jam, cold hot fudge ice cream topping, or cold caramel ice cream topping. If desired, melt some dark or white baking chocolate to drizzle over the top.: Cooling is when the cookies finish setting, and they firm up while maintaining softness. Leaving them on the sheet allows gentle residual heat to finish the texture without crisping the bottoms. Trying to lift them too soon can cause them to break apart, so patience pays off here. If they stick, a thin offset spatula can help, but waiting is usually the simplest solution.
Store in an airtight container for up to 3 days or freeze for up to one month.: Proper storage preserves that soft texture, and the lack of air prevents staling. For freezing, I prefer to flash freeze on a tray then transfer to a sealed bag so they keep their shape and don’t clump together. A pitfall is storing while still warm, which creates condensation and soggy cookies; always cool fully before sealing.
For M&M Shortbread Cookies: add M&Ms on top of the cookie dough ball before baking. : The candy will sink slightly as the cookie spreads and the colors create a cheerful surface. For two tablespoon sized cookies, placing five to six M&Ms gives a nice evenly colored crown. Press too hard or use too many candies and they can weigh the cookie down, causing uneven browning and a denser center.
For Chocolate Dipped Shortbread Cookies: melt 4 ounces of white or semi-sweet baking chocolate according to package directions. : Melted chocolate brings a glossy, crisp contrast to the soft shortbread. Dip cooled cookies halfway, tap off excess, then chill to set, and the texture contrast is heavenly. Work gently and avoid overheating the chocolate, which can become grainy; use gentle heat or a double boiler for stability.
For Thumbprint Cookies: make 1 tablespoon sized balls of cookie dough. Roll them between your hands to form a ball. Use the back of a wooden spoon to slightly make an indent in the center of each cookie. : Creating the indent while the dough is still soft ensures a tidy well for jam or caramel, and re pressing after baking preserves an even cavity if it puffed slightly. Bake until lightly golden on the bottom and then re press the hole as needed, then fill with about a half teaspoon of filling. The key mistake to avoid is making the indent too deep, which can thin the base and cause spreading or leaks during baking.
If desired, drizzle with melted chocolate after filling.: A final drizzle adds visual polish and a textural crisp that pairs beautifully with the soft center. Use a fork or piping bag to create thin ribbons, and let the drizzle set fully on a lined tray. Overdrizzling can overwhelm the delicate shortbread, so a light hand achieves the best balance.