Place a rack in center of your oven and preheat the oven to 350°F. Generously grease and flour a mini-muffin tin. Or line with paper liners and coat the liners with nonstick spray (you’ll thank yourself later).

Place the butter, sugar, cocoa powder, and salt in a heat-proof bowl, then set the bowl over a pan of simmering water to create a double boiler ensuring that bottom of bowl and water do not touch. Heat until butter melts, stirring occasionally. This will take several minutes. Carefully remove the bowl from pan and stir to combine as best you can. Set aside and let cool until still warm to the touch, but not hot. The batter will be very gritty but will smooth out as you continue with recipe.

Add the vanilla extract and espresso powder. With a wooden spoon or heatproof spatula, stir to combine.

Add eggs one at a time, stirring thoroughly after each addition so that the eggs are fully combined before you add the next ingredient. Keep on stirring until batter looks thick and shiny.

Sprinkle the flour over the top. Stir until it disappears into batter.

As soon as you can no longer see the flour, stir the batter for 40 additional turns of the spatula (a.k.a. strokes). It will look thick and glossy.

With a small cookie scoop or spoon, dollop the batter into mini muffin tins, filling each about 3/4 of way to top. Add any additional toppings you might like (if doing peanut butter or caramel cups, unwrap and carefully place one in the center of each brownie bite).

Bake the brownie bites are just set, about 12 minutes. Place the pan on a wire rack and let cool in pan for 15 minutes. With a fork, carefully loosen the brownie bites and transfer to a wire rack to cool completely.