Preheat your oven to 350°F (175°C) and generously grease a 24-cup mini muffin tin with cooking spray or butter. Don’t skip this step—properly greased cups ensure your brownie bites release cleanly.
Melt the butter and chocolate together in a microwave-safe bowl in 30-second intervals, stirring between each until completely smooth. Be careful not to overheat—stop microwaving when a few small chocolate pieces remain and stir until they melt from the residual heat.
Whisk the sugar into the chocolate mixture until well combined, then let it cool for about 5 minutes. The mixture should be warm but not hot to prevent cooking the eggs in the next step.
Add the eggs one at a time, whisking well after each addition until fully incorporated. Then stir in the vanilla extract until the mixture becomes shiny and smooth.
In a separate bowl, whisk together the flour, cocoa powder, salt, and espresso powder (if using) to remove any lumps.
Gently fold the dry ingredients into the wet ingredients using a spatula, mixing just until no flour streaks remain. Overmixing will lead to dense, tough brownies.
Fold in the chocolate chips or chunks, distributing them evenly throughout the batter.
Fill each mini muffin cup about ¾ full with batter. An ice cream scoop or small cookie scoop works perfectly for this task, ensuring even-sized bites.
Bake for 12-15 minutes until the tops are set and a toothpick inserted comes out with a few moist crumbs (not wet batter). The secret to fudgy brownies is slight underbaking—they’ll continue cooking from residual heat.
Allow the brownie bites to cool in the pan for 5 minutes, then carefully run a butter knife around the edges and gently lift them out to cool completely on a wire rack.