Prep your pan and oven. Heat oven to 350°F (175°C). Line a 12-inch pizza pan or a large round cake pan with parchment and lightly grease the sides. No pizza pan? Use a 9x13-inch sheet pan for a rectangle “pizza.”
Cream the butter and sugar. In a large bowl, beat 3/4 cup unsalted butter with 1 cup granulated sugar until pale and fluffy, about 2–3 minutes. Add 1 large egg, 2 teaspoons vanilla, and 1/4 teaspoon almond extract (optional). Beat until smooth.
Combine dry ingredients. Whisk 2 1/4 cups all-purpose flour, 1 teaspoon baking powder, 1/2 teaspoon kosher salt, and 1 tablespoon cornstarch (optional) in a separate bowl. The cornstarch helps the cookie stay soft.
Make the dough. Add dry ingredients to the butter mixture and mix on low just until combined. If you like a citrus twist, fold in 1 teaspoon lemon or orange zest now.
Press and smooth. Transfer dough to the pan. Press it into an even layer, leaving a tiny border from the edge to allow for spreading. Smooth with a spatula or the back of a measuring cup.
Bake. Bake 12–16 minutes until the edges are set and lightly golden and the center looks just dry. Do not overbake. The cookie will continue to set as it cools, staying soft and tender.
Cool completely. Let the cookie cool in the pan for 15 minutes, then lift out using the parchment and cool fully on a rack. Frosting a warm cookie will cause slipping and melting.
Make the frosting. Beat 1/2 cup unsalted butter (room temp) until creamy, 1–2 minutes. Add 2 1/2–3 cups powdered sugar, 1 teaspoon vanilla, a pinch of salt, and 2–3 tablespoons heavy cream or milk. Beat on medium-high until fluffy, 2–3 minutes. For a tangy option, replace half the butter with 4 ounces softened cream cheese and whip until smooth before adding sugar.
Tint to pastel. Divide frosting into bowls. Add the tiniest amount of gel or liquid food dye to create soft pastel shades. Stir well. If frosting gets too thin, add more powdered sugar; if too thick, add a splash of cream.
Decorate. Spread a base layer of one pastel color over the cooled cookie. Add swoops of the other colors on top and blend gently with an offset spatula for a marbled look. Finish with pastel sprinkles. Keep it simple and let the colors do the work.
Slice and serve. Use a large knife or pizza cutter. For clean slices, chill the frosted cookie 10–15 minutes first.