Heat the oven to 350°F. Line a 9×13 pan with parchment paper and spray lightly with baking spray.
In a large bowl, whisk together the melted butter, brown sugar, and granulated sugar until smooth. Add eggs plus yolk and vanilla. Whisk until glossy.
Stir in flour, baking powder, and salt until just combined- don’t overmix. The batter will be thick.
Split the batter evenly into three bowls. Tint one pink, one teal, and one yellow with gel food coloring.
Drop spoonfuls of each color randomly into the pan, patchwork style. Use a knife or spoon to gently smooth the top.
Bake for 25–30 minutes, or until a toothpick comes out with moist crumbs (not wet batter). Cool completely in the pan.
Melt the dark chocolate in 30-second microwave intervals until smooth. Transfer to a piping bag or bottle with a small tip. Pipe stitches across the type of the bars and let them set completely.