Prep the pan: Line an 8x8-inch pan with parchment paper, leaving overhang for easy lifting. Lightly grease the parchment if your mix tends to stick.
Mix the dry ingredients: In a large bowl, stir together oats, chocolate protein powder, cocoa powder, and salt until evenly combined.
Warm the wet ingredients: In a small saucepan over low heat, gently warm the nut butter, honey/maple, coconut oil, and vanilla until smooth and pourable. Don’t boil; you just want it silky.
Bring it together: Pour the warm mixture over the dry ingredients.
Add half the milk and stir. Continue adding milk a little at a time until the mixture is thick, slightly sticky, and holds together when pressed. It should be moldable, not crumbly or runny.
Fold in extras: If using chocolate chips or nuts, let the mixture cool a minute so the chips don’t melt, then fold them in.
Press into the pan: Transfer the mixture to the lined pan. Press firmly and evenly with a spatula or the back of a measuring cup. Pack it down well to help the bars hold their shape.
Optional chocolate topping: Melt the dark chocolate with 1 teaspoon coconut oil in short microwave bursts (or on the stovetop).
Spread a thin layer over the bars.
Chill to set: Refrigerate for 1–2 hours, or until firm. For faster setting, freeze for 25–30 minutes.
Slice and store: Lift the slab out using the parchment. Cut into 10–12 bars with a sharp knife.
Wipe the blade between cuts for clean edges.