Preheat the oven to 350°F (180°C).
In a microwave-safe bowl, melt the dark chocolate and butter in 30-second bursts, stirring after each, until smooth.
Stir in the cocoa powders.
In a stand mixer fitted with a whisk attachment, or using an electric whisk, whisk the egg, white sugar, and brown sugar until light and fluffy (about 5 minutes on medium-high).
Fold the chocolate mixture and the egg mixture together until smooth.
Fold in the flour and salt until just combined.
Divide the batter evenly among the cavities of your silicone mold, filling each heart well nearly to the top. Any additional batter can be scooped onto a parchment lined cookie sheet and baked as brownie cookies!
Bake for 12-15 minutes, until the brownies have set and a toothpick inserted into the centre comes out with just a few crumbs.
Let the brownies cool completely before removing from the mold.
Wash out the silicone mold, and dry it completely before moving on to the next step.