Preheat and prep: Set the oven to 350°F (175°C).
Line a baking sheet with parchment paper. This prevents sticking and helps the cookies bake evenly.
Mix dry ingredients: In a medium bowl, whisk together almond flour, cocoa powder, erythritol, baking powder, salt, and espresso powder (if using). Break up any cocoa clumps with the whisk.
Combine wet ingredients: In a separate bowl, whisk the melted butter, egg, egg yolk, and vanilla until smooth.
Make sure the butter is not hot, or it can scramble the egg.
Bring the dough together: Pour the wet mixture into the dry ingredients. Stir with a spatula until thick and glossy. If the dough is crumbly, add almond milk 1 tablespoon at a time until it looks like brownie batter but holds a scoopable shape.
Fold in chips: Gently mix in the sugar-free chocolate chips.
The dough should be soft and slightly sticky.
Scoop and shape: Use a medium cookie scoop (about 1 1/2 tablespoons) to portion dough onto the prepared sheet, spacing about 2 inches apart. Lightly flatten the tops with damp fingertips. Press a few extra chips on top for looks.
Bake: Bake for 9–11 minutes, until the edges look set but the centers are still soft and slightly puffy.
Do not overbake; they will firm up as they cool.
Cool: Let cookies cool on the sheet for 10 minutes, then move to a rack. For the fudgiest texture, allow them to cool completely. Sprinkle with a pinch of flaky sea salt if you like.