Preheat the oven to 350°F (175°C) and line a small baking sheet with parchment.
For a faster option, you can use the microwave (see FAQ).
Mix dry ingredients. In a small bowl, stir together protein powder, oats or flour, baking powder, and a pinch of salt until evenly combined.
Add wet ingredients. Stir in nut butter, sweetener, milk, and vanilla. Mix with a spoon until a thick dough forms. If it’s crumbly, add milk 1/2 teaspoon at a time; if it’s sticky, add a pinch more oats or flour.
Fold in chocolate. Add chocolate chips if using, saving a few to press on top.
Shape the cookie. Scoop the dough onto the prepared sheet and flatten to about 1/2 inch thick.
Press on the remaining chips.
Bake. Bake for 8–10 minutes until the edges are set and the center is just slightly soft. It firms up as it cools.
Cool briefly. Let the cookie rest on the sheet for 3–5 minutes, then enjoy warm or transfer to a rack to cool fully.