Preheat the oven to 350 degrees. Use the convection bake setting if you have it.
Grease and line one 9-inch round cake pan and one 9x9-inch baking pan with parchment paper (if you don't have parchment paper, just grease well). Set the pans aside.
With a hand or stand mixer, mix the melted butter and sugar for 1-2 minutes on medium speed.
Add the eggs, vanilla extract, melted chocolate and salt. Mix until combined.
Add the cocoa powder and mix (carefully) until well combined and scrape down the bowl.
Lastly, add the flour and mix until well-combined.
Split the batter evenly among the two prepared cake pans. Use a kitchen scale to measure more accurately, if desired.
Bake for 20-25 minutes, or until a toothpick inserted in the center of the pan comes with just a few crumbs. I prefer to remove the brownies before the toothpick comes out completely clean so that you maintain that fudgy texture!
Once fully baked, remove the brownies from the oven and allow them to cool to room temperature before removing them from the pans and assembling the heart brownie. Put them in the freezer to cool more quickly, if desired.