Preheat the oven to 350°F. Grease and flour two 9-inch round pans.: Warmth spreading through the oven signals readiness, and that first breath of heat has a dry, neutral smell. Greasing and flouring ensures the layers unmold cleanly and maintain their shape; a properly prepared pan will release the cake with smooth sides. Press a little flour into any oily spots to avoid bare patches that can stick, and tap out excess flour so the crust does not become gritty. A frequent mistake is under prepping the pans, which leads to tearing or crumbling when you try to remove layers. Check for even oven temperature by placing a thermometer inside if your oven runs hot or cool.
Sift together the flour, baking powder and salt. Set aside.: The sifted flour mixture has a light, airy texture and a faint wheaty scent, and sifting breaks up clumps so the leavening disperses evenly. Mixing these dry ingredients first prevents pockets of baking powder that could cause uneven rise or metallic flavor. If you skip sifting, you may end up with dense spots in the crumb, especially where baking powder lumps occur. A good test is to whisk until the mixture looks uniform and smooth, with no visible clumps.
In a large bowl, use an electric mixer to beat the sugar, shortening, eggs and orange zest at high speed for 3 minutes. Beat in the flour mixture a little at a time, alternating with orange juice and milk, beating after each addition until the batter is smooth.: As you beat, the mixture will lighten in color and take on a billowy texture, releasing bright orange perfume from the zest. This creaming step traps tiny air bubbles that expand in the oven, contributing to lift, and the sound will be a steady hum with occasional peaks as volume builds. Underbeating leaves the batter heavy, while overbeating can cause too much air and a coarse crumb. Keep an eye on the mixture, and scrape the bowl sides once during the time to ensure everything is evenly incorporated.
Divide the batter between the prepared pans. Bake for 30 minutes, or until a toothpick inserted into the center comes out clean. Let cool, and then frost with orange buttercream frosting.: The batter will feel silky and slightly thick, with the citrus aroma deepening as juice joins the mix. Alternating wet and dry ingredients helps maintain emulsion and prevents the batter from splitting, producing a uniform crumb when baked. If you add all the liquid at once you risk a runny batter and uneven texture, while adding all the flour at once can make it lumpy and overworked. Watch for the batter to come together smoothly without streaks of flour, and be careful not to overmix once it becomes homogeneous.
Use an electric mixer to combine the butter, zest, salt and egg yolk in a medium bowl. Add the sugar alternately with orange juice beating until of spreading or piping consistency.: You will hear the oven settle into a quiet roar as the cakes rise, and the tops should turn a light golden hue with a tender spring when pressed gently. The toothpick test confirms doneness; it should come out with a few moist crumbs but no raw batter. Cooling is crucial because warm cake will melt the frosting, so transfer layers to a rack and let air circulate underneath. A common mistake is frosting too soon, which results in runny edges and uneven layering. Wait until the layers are fully cool to the touch before you assemble and ice them.
Use an electric mixer to combine the butter, zest, salt and egg yolk in a medium bowl. Add the sugar alternately with orange juice beating until of spreading or piping consistency.: As you whip, the butter will become glossy and the mixture fragrant with orange oils, and the rhythm of the mixer changes as the texture moves from chunky to creamy. Adding powdered sugar in portions lets you control the frosting thickness, and alternating with orange juice ensures a smooth, glossy finish that is easy to spread or pipe. If you rush and dump everything in, the frosting can split or become overly runny; if it feels too stiff, add a teaspoon of juice at a time. Watch for a silky mouthfeel and shine, and taste to balance sweetness with citrus tang. A typical error is not fully incorporating the yolk, which can leave small pockets that affect texture, so scrape the bowl sides and beat until smooth.