Preheat oven to 350 degrees. Line a 9 x 13 inch cake pan with parchment, with some of the paper overhanging the edges.
Melt the butter and chocolate chips together for one minute. Stir until the bowl is no longer warm. If needed, microwave 20-30 seconds longer until smooth. Whisk in the sugar and brown sugar.
In a large bowl, beat the eggs until very frothy, about 2-3 minutes. With the mixer running, pour in the butter-chocolate mixture and the vanilla and beat until incorporated. Add the flour, cocoa powder, baking powder, and salt. Stir until just combined.
Spread the batter into the prepared cake pan. Bake 22-25 minutes until an inserted toothpick emerges with just a few crumbs. Cool in the pan 15 minutes, then transfer the pan to the fridge for at least an hour to chill.
When ready to decorate, make the ganache. Microwave the chocolate chips for 1 minute. Stir. Heat an additional 20-30 seconds until melted and smooth. Whisk in the heavy cream and vanilla until very silky.
Using a cookie cutter, stamp out heart shapes from the chilled brownies. The smaller the cutout, the more brownies you will get. Dip the crinkly side of the brownie into the ganache. I like the swirls, but if you'd like a smooth topping, spread the ganache with a spatula. Dot the ganache with nonpareils. Chill the brownies another 30 minutes.