In a medium bowl, whisk together flour, baking powder, baking soda and salt.
2½ cups all purpose flour, 1 teaspoon baking powder, 1/2 teaspoon baking soda, Pinch salt
Add dry mixture to wet mixture and mix until a soft dough is formed. It will be heavy!
Spread the dough evenly in the prepared baking dish and bake for 20 minutes, or until a toothpick inserted in the middle comes out clean or with only a few moist crumbs.
Let cool in pan for 5 minutes, then lift out of the pan by the parchment paper and let cool completely on a cooling rack.
Frosting
In a large bowl, use the paddle attachment on your mixer on high speed to beat the butter until smooth.
3/4 cup unsalted butter
Add butter emulsion and then the powdered sugar, a little at a time, and beat on high speed until light and fluffy. Add heavy cream, one tablespoon at a time, if needed to get the desired consistency.
Top with as many rainbow nonpareils as your heart desires. Cut into triangles, serve, and enjoy!
Rainbow nonpareils
Notes
How to cut for serving: - Since these are dense, you'll want them in small triangles. To do that, start by cutting the bars into 2 rows of 4 rectangles. - Next, cut each of those 8 rectangles diagonally to form 16 triangles. - Then cut each of the rectangles again, diagonally in the opposite direction. - You should end up with 32 small triangles that are perfect for serving!Storage: Best served same day, but can be kept in the fridge for up to 5 days in an airtight container. The sprinkles will bleed into the frosting a bit as time goes by.