Prep the pan and oven. Heat the oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment, leaving overhang on the long sides. Lightly grease the parchment.

Whisk dry ingredients. In a bowl, combine 2 3/4 cups all-purpose flour, 2 tablespoons cornstarch, 1 teaspoon baking powder, and 1/2 teaspoon fine salt. Set aside.

Cream the butter and sugar. In a large bowl, beat 1 cup (2 sticks) unsalted butter with 1 1/4 cups granulated sugar on medium-high until pale and fluffy, about 2–3 minutes. Don’t rush this—creaming helps with lift and texture.

Add eggs and flavor. Beat in 2 large eggs, one at a time, scraping the bowl. Mix in 2 teaspoons vanilla extract and 1/4 teaspoon almond extract if using.

Bring the dough together. Add the dry ingredients in two additions, mixing on low just until combined. The dough will be thick and soft. Avoid overmixing.

Press and level. Transfer dough to the pan. Use lightly greased hands or an offset spatula to press it into an even layer, reaching all corners. Aim for a smooth, level surface.

Bake. Bake 16–20 minutes, until the edges are lightly golden and the center looks set. A toothpick should come out with a few moist crumbs, not wet batter. Don’t overbake—this is key for soft bars.

Cool completely. Set the pan on a rack and cool fully. Frosting a warm base will cause melting and sliding.

Make the frosting. Beat 3/4 cup (1 1/2 sticks) unsalted butter until creamy. Gradually add 3 cups powdered sugar, 1 teaspoon vanilla extract, a pinch of salt, and 2–3 tablespoons milk or cream. Beat on medium-high until light and fluffy, 2 minutes. Adjust milk to reach a spreadable, not runny, consistency.

Frost and decorate. Spread frosting over the cooled bars. Add sprinkles if you like. For cleaner cuts, chill the frosted pan 15–20 minutes.

Slice and serve. Use the parchment overhang to lift the slab out. Cut into 16 large bars or up to 24 smaller squares with a sharp knife, wiping the blade between cuts.