Preheat the oven to 350°F (175°C). Position the rack in the middle of the oven. Line a 9×13-inch baking pan with parchment paper, leaving overhang on the long sides like handles, and lightly grease the parchment and exposed pan edges.

In a large mixing bowl, beat 1 cup softened unsalted butter, granulated sugar, and light brown sugar together on medium speed for 2–3 minutes, until light and fluffy and slightly paler in color.

Add the eggs one at a time, beating well after each addition and scraping down the bowl as needed. Mix in 2 teaspoons vanilla extract and the almond extract (if using) until smooth and thick, like fluffy cake batter.

In a separate bowl, whisk together the flour, cornstarch, baking powder, baking soda, and fine sea salt until well combined.

Gradually add the dry ingredients to the wet mixture on low speed (or with a spatula), mixing just until no dry streaks remain. The dough will be thick but soft. Gently fold in the 3/4 cup rainbow jimmies sprinkles until evenly distributed without overmixing.

Transfer the dough to the prepared pan. Use a spatula or clean, lightly damp or greased hands to press it evenly into the pan, making sure to reach the corners. Bake for 18–24 minutes, until the edges are lightly golden, the top looks set (not shiny), and a toothpick near the center comes out with a few moist crumbs but no wet batter. Do not overbake.

Place the pan on a wire rack and let the cookie bar base cool completely in the pan, at least 45–60 minutes. The bars will finish setting as they cool; frosting them while warm will melt the frosting.

While the bars cool, beat 3/4 cup softened unsalted butter in a medium bowl for 1–2 minutes until smooth and creamy. Gradually add the powdered sugar, about 1 cup at a time, mixing on low at first and then on medium until combined.

Add 2 tablespoons heavy cream or whole milk, 2 teaspoons vanilla extract, and a pinch of salt to the frosting. Beat on medium-high for 2–3 minutes until light, fluffy, and spreadable. If too thick, add more cream or milk 1 teaspoon at a time; if too thin, add a bit more powdered sugar.

Once the cookie bars are completely cool, spread the frosting evenly over the surface with an offset spatula or the back of a spoon. Immediately shower the top with 1/4–1/2 cup rainbow sprinkles, gently pressing them into the soft frosting so they adhere.

For cleaner cuts, chill the frosted bars for 20–30 minutes. Use the parchment overhang to lift the entire slab out of the pan onto a cutting board. Slice into 20–24 bars with a sharp knife, wiping the blade with a warm, damp cloth between cuts if you want very neat edges. Serve at room temperature.

Store frosted bars in an airtight container at room temperature for up to 2 days if your kitchen is cool, or in the refrigerator for up to 5 days (bring to room temperature before serving). To freeze, chill until firm, then wrap the slab or individual bars tightly and freeze for up to 2 months. Thaw overnight in the fridge, then bring to room temperature before serving.