Preheat your oven to 350°F. While it heats, spray two mini muffin pans thoroughly with cooking spray, making sure to coat all the indentations evenly so the brownie bites release cleanly after baking.

In a medium bowl, whisk together the flour, sugar, cocoa powder, instant espresso powder, baking soda, and salt. Make sure to break up any cocoa clumps and distribute the espresso powder evenly throughout—this ensures consistent flavor and rise in every bite. The espresso powder won't be noticeable as coffee flavor, but rather deepens the chocolate richness, which is exactly what we want in these Irish coffee-inspired treats.

In a separate small bowl, whisk together the melted and cooled butter, whiskey, egg, and vanilla extract until well combined. Pour this wet mixture into the dry ingredients from Step 2 and fold together with a spatula until just combined—the batter should be thick and fudgy, not overmixed. I prefer to fold rather than stir vigorously because it keeps the texture tender and prevents overdeveloping the gluten, which would make the bites cake-like instead of chewy.

Divide the batter into 20 portions, rolling each into a small ball about the size of a walnut. Place one ball into each prepared muffin cup in the two pans. Don't worry if they're not perfectly round—they'll spread slightly during baking and fill out the muffin cup shape naturally.

Place both muffin pans in the preheated 350°F oven and bake for 12–15 minutes, until a toothpick inserted into the center of one brownie bite comes out with a few moist crumbs still clinging to it. The tops should look set but still have a slight dome or slight crack—this indicates they're fudgy inside, not overdone and dry.

Remove the pans from the oven and let them cool on the counter for about 5 minutes. This brief rest helps them set just enough to handle. Run a small knife around the edge of each brownie bite to loosen it from the pan, then gently pop them out onto a wire cooling rack. Let them cool completely before serving—they'll continue to firm up as they cool, making them easier to handle.