Mix dry ingredients: In a large bowl, stir together oats, protein powder, cocoa powder, salt, and espresso powder (if using).
Add wet ingredients: Stir in peanut butter, honey, and vanilla. Mix well until a thick, dough-like mixture forms.
Adjust texture: If the mixture is too dry, add 1 tablespoon of milk or water at a time until it sticks together easily.
Fold in mix-ins: Stir through mini chocolate chips or nuts if using.
Shape balls: Scoop about 1 tablespoon of dough per ball and roll between your hands to form smooth balls.
Chill: Place the balls on a tray and refrigerate for 20 minutes to firm up before serving.