Preheat the oven to 180°C (350°F) with a rack in the centre. Line a baking sheet with parchment paper.
In a bowl, stir the peanut butter, egg, maple syrup and vanilla until smooth. If the peanut butter is very stiff, warm it in the microwave for 15 seconds first.
Add the protein powder, oat flour, baking powder and salt and stir into a thick dough. Fold in the chopped chocolate.
Roll into 12 balls, about 2 tablespoons each, and space them 5 cm (2 inches) apart on the tray. Flatten each to about 1.5 cm (1/2 inch) with a fork, making a criss-cross.
Bake on the centre rack for 9 to 10 minutes, until the edges are set and lightly golden but the centres still look soft.
Cool on the tray for 10 minutes; they firm up as they cool. Then move to a rack. Keeps 5 days sealed.
Tips
Pull them while they still look soft in the middle.