Brown the butter by placing it in a medium-sized saucepan and melting over medium-low heat.
Once butter has melted, increase heat to just above medium. Stir constantly with a wooden spoon as it bubbles, pops and foams. Once the bubbling slows and the liquid begins to turn brown (you’ll notice golden flakes of cooked butter), remove from heat immediately and pour into a large, heatproof bowl.
Allow to cool 25 minutes before proceeding.
Preheat oven to 350F (175C) and line a 13x9-inch pan with parchment paper.
Combine browned butter and sugars, stir well.
Add eggs and egg yolk, stirring well after each addition. Stir in vanilla extract.
In a separate bowl, whisk together flour, cornstarch, baking powder, and salt.
Gradually stir dry ingredients into wet until completely combined.
Spread batter evenly into prepared pan. Smooth the surface with a spatula or knife so that it doesn’t have any craggy pieces when baked (I like to spread it into the pan, then lay a piece of wax paper over the surface and use my hands to smooth the surface, then discard the wax paper).
Bake on 350F (175C) for 18-23 minutes -- until edges just begin to turn golden brown and a toothpick inserted in the center comes out clean or with few crumbs. Allow to cool completely.
Once cooled, lift from pan (using parchment paper) and transfer to cutting board -- cut into 1” squares (I cut 13 rows down the longer end and 9 down the shorter end -- super simple!).
Prepare melting wafers according to package instructions in shallow bowls (keep white and dark chocolate in separate bowls).
Dip the tops or bottoms (I alternated) of the butter bites in chocolate and place on wax paper. Immediately sprinkle with nonpareils, if desired.
Allow chocolate to harden before enjoying (just a few minutes!).