Prep the pan: Heat oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners, or grease well.
Mix dry ingredients: In a large bowl, whisk almond flour, cocoa powder, sweetener, baking powder, salt, and espresso powder (if using) until no lumps remain.
Combine wet ingredients: In a separate bowl, whisk eggs, almond milk, yogurt or sour cream, melted butter or oil, and vanilla until smooth.
Bring it together: Pour wet ingredients into the dry. Stir gently with a spatula until just combined.
The batter will be thick but scoopable. Fold in most of the chocolate chips, saving a few for the tops.
Fill the cups: Divide batter evenly among the muffin wells, filling each about 3/4 full. Sprinkle the remaining chocolate on top.
Bake: Place on the center rack and bake 18–22 minutes, or until the tops are set and a toothpick comes out with a few moist crumbs (melted chocolate is fine).
Cool: Let muffins cool in the pan for 5 minutes, then transfer to a wire rack.
They firm up as they cool.
Serve: Enjoy warm for the melty chocolate experience, or at room temperature for a neater bite.