Preheat oven to 175°C (350°F). Grease a silicone brownie bites mold or square pan.
In a heatproof bowl, melt butter and dark chocolate together using a double boiler or microwave (30-second intervals, stirring between). Let cool slightly.
In a mixing bowl, whisk together eggs and sugar until well combined, about 1-2 minutes. Stir in vanilla extract.
Sift together flour, cocoa powder, and salt. Gently fold dry ingredients into egg mixture in two additions, just until combined. Don't overmix.
Pour melted chocolate mixture into batter and stir until fully combined and glossy.
Transfer batter to a piping bag or use a spoon to fill molds ⅔ full.
Bake for 10-15 minutes, until a toothpick inserted comes out with moist crumbs (not raw batter). Start checking at 10 minutes.
Cool in mold for 5 minutes, then transfer to a wire rack to cool completely before decorating.