Preheat your oven to 350°F (177°C). Line a 9 x 9-inch (23 x 23 cm) square metal baking pan with parchment paper, leaving an overhang on two sides so you can lift the bars out later. Press the parchment neatly into the corners. The pan should be fully lined across the bottom, with the paper lying reasonably flat.

In the medium mixing bowl, add the following ingredients from the Sugar Cookie Bars section, keeping each measured amount exactly as listed: 1 1/2 cups (188 g) all-purpose flour, spooned and leveled; 1 teaspoon (4 g) baking powder; 1/4 teaspoon (1 g) baking soda; 1 1/2 teaspoons (4 g) cornstarch; and 1/4 teaspoon (1.5 g) fine salt. Whisk until all the ingredients are evenly distributed and no visible lumps of cornstarch remain. Set the bowl aside while you prepare the butter mixture.

Add the following ingredients to the large mixing bowl: 1/2 cup (113 g) unsalted butter, softened to room temperature; and 3/4 cup (150 g) granulated sugar. Beat with an electric mixer on medium-high speed for about 3 minutes, scraping down the bowl as needed. The mixture should look lighter in color, creamy, and slightly fluffy rather than dense or oily.

Add 1 large egg (approximately 50 g without shell), at room temperature, 1 large egg yolk (approximately 18 g), at room temperature, and 2 teaspoons (10 mL) pure vanilla extract to the creamed butter mixture. Beat on medium speed until everything is evenly combined, stopping to scrape the bowl. The mixture should look thick and creamy. A slightly uneven appearance at first is normal; continue mixing until the egg and egg yolk are fully incorporated.

Add the prepared dry ingredient mixture to the wet ingredients. Mix on low speed until the flour is mostly incorporated, then briefly increase to medium speed until no dry flour remains. Measure the following ingredient exactly as listed: 6 tablespoons (approximately 60 g) red and green jimmies-style sprinkles, for mixing into the dough. Fold the measured sprinkles into the dough using a silicone spatula. The finished dough should be thick, soft, and slightly sticky, with the sprinkles distributed throughout.

Transfer the cookie dough into the prepared 9 x 9-inch (23 x 23 cm) square metal baking pan. Use the silicone spatula to press it into an even layer, making sure the dough reaches the corners. Bake at 350°F (177°C) for 25–26 minutes, checking the edges near the end. The bars are ready when the edges look very lightly golden and the center appears set but still soft. Avoid waiting for the entire top to become brown, which can make the finished bars dry.

Remove the pan from the oven and place it on a wire cooling rack. Allow the cookie layer to cool completely, approximately 60 minutes, before adding the frosting. The top should feel cool, and the center should be firm enough to support the frosting without crumbling. Don&x27;t worry if the middle sinks slightly during cooling; that&x27;s normal for soft sugar cookie bars. If the bars are still warm after the cooling period, continue cooling until they reach room temperature.

Place 3/4 cup (170 g) unsalted butter, softened to room temperature, in the large mixing bowl after cleaning and drying it. Beat on medium speed for about 2 minutes until smooth and creamy. Add the following ingredients from the Vanilla Buttercream Frosting section, keeping the specified amounts and ingredient states unchanged: 2 1/4 cups (270 g) confectioners&x27; sugar, sifted; 3 tablespoons (45 mL) heavy whipping cream, cold; 1 teaspoon (5 mL) pure vanilla extract; and 1/8 teaspoon (approximately 0.75 g) fine salt. Mix on low speed for about 30 seconds, then increase to medium-high speed and beat for approximately 3 minutes. The frosting should look smooth, light, and fluffy, with a consistency that spreads easily but holds its shape.

Lift the completely cooled cookie layer from the pan using the parchment overhang and place it on a cutting board. Spread the prepared vanilla buttercream evenly over the top with an offset spatula. Measure the final ingredient exactly as listed: 2 tablespoons (approximately 20 g) red and green Christmas sprinkles, for decorating the frosting. Scatter the measured sprinkles evenly over the frosting before the surface begins to set. Cut the slab into 16 equal bars using a sharp knife, wiping the blade clean between cuts. The finished bars should have tidy edges, a tender cookie base, and a smooth layer of frosting.