In a microwave-safe mug or small bowl, mash the avocado until completely smooth—this creates the fudgy, creamy texture that makes these brownies special. Add the egg yolk, coconut oil, liquid stevia, and vanilla extract, then whisk together until well combined and relatively uniform in texture. I like to use an avocado that's perfectly ripe but not overly soft, as this gives you better control over the final consistency.

Sift the cocoa powder, almond flour, baking soda, salt, and espresso powder into the wet mixture. Fold gently with a spatula or fork until just combined—don't overmix, as this can make the brownies tough. The espresso powder enhances the chocolate flavor without making it taste like coffee, and I find it creates a deeper, more sophisticated chocolate taste.

Microwave the batter on HIGH for 90 seconds until the top looks mostly set but still has a slight shimmer of moisture—it should spring back slightly when lightly touched. Remove from the microwave and immediately sprinkle the chocolate chips over the top while the brownie is still hot so they soften slightly. Let it cool for 1 minute before eating, as the interior will still be warm and fudgy. If you prefer a less dense texture, you can bake this in a small ramekin at 350°F for 15-20 minutes instead.