Prep time: 20 minutes
Total time: 40 minutes
4 tablespoons unsalted butter, room temperature, plus more for tin
⅓ cup sugar, plus more for tin
¼ ounces bittersweet chocolate, coarsely chopped
⅓ cup all-purposed flour
1 tablespoon instant espresso powder
¼ teaspoon coarse salt
3 large eggs
1 teaspoon pure vanilla extract
Preheat oven to 400º. Brush 6 cups of a standard muffin tin with butter and dust with sugar; tap out excess. Melt chocolate in a heat-proof bowl set over (not in) a pan of simmering water, stirring until smooth. Let cool.
In a bowl, whisk together flour, espresso powder, and salt, with a mixer on medium-high speed, beat butter with sugar until light and fluffy. Add eggs, one at a time, mixing until completely incorporated. Add flour mixture; beat until combined. Beat vanilla and chocolate.
Spoon batter into prepared cups. Bake until cakes no longer jiggle when tin is shaken, 8 to 10 minutes. Let cool on a wire rack 10 minutes before turning out cakes.