Preheat the oven to 350 degrees F (175 degrees C). Line a 9×13″ baking pan with parchment paper, and spray parchment paper with non-stick spray.
Mix sugar, flour, cocoa powder, salt, and baking powder together in a mixing bowl.
Mix butter, eggs, and vanilla extract together in a separate bowl; add butter mixture to the bowl with the flour mixture and stir batter until well mixed.
Pour the batter into the prepared baking pan.
Bake in the preheated oven until edges are brown and center is set, about 30 minutes.
Remove pan from the oven and let brownies cool completely, about 30 minutes.
Remove the brownies from the pan by lifting the parchment paper (with brownies) out of the pan. Using a small heart-shaped cookie cutter, cut hearts out of the brownies, then set them onto a large piece of waxed paper.
One color at a time, melt the candy melts according to package directions. Place melted candy in a plastic zipper bag, and cut off a very small bit of one of the tips of the bag. Pipe zig-zag stripes over 1/3 of the brownies. Repeat with the other two colors until all brownies are decorated.
Once the candy melts have cooled and hardened, break off any excess around the edges of the hearts, and serve.