01 -
Preheat the oven to 350°F (175°C). Generously grease a 12-cup muffin pan with softened butter, then dust with cocoa powder and tap out any excess.
02 -
In a medium saucepan over low heat, melt the butter and chopped chocolate together, stirring constantly until smooth. Remove from heat and cool for 5 minutes.
03 -
Whisk the sugar into the chocolate mixture. Add the eggs and egg yolk one at a time, whisking well after each. Stir in the vanilla. The batter will thicken and look glossy.
04 -
Sift the flour, cocoa powder, and salt over the batter. Fold with a spatula until just combined, stopping as soon as no dry streaks remain. Fold in any mix-ins if using.
05 -
Divide the batter evenly among the 12 muffin cups, filling each about two-thirds full. A cookie scoop keeps portions even and speeds things up.
06 -
Bake for 18 to 22 minutes until the edges pull away from the cup sides, the tops look set and dry, and a toothpick inserted in the center comes out with a few moist crumbs, not wet batter. Pull them on the earlier side for a fudgier center.
07 -
Let the pan cool on a wire rack for 10 minutes. Run a thin knife or small offset spatula around each cup and lift the brownies out. They firm up as they cool.