Line an 8x8-inch pan with parchment paper, leaving overhang for easy lifting. Lightly mist with nonstick spray if you like.

In a medium bowl, whisk together the melted coconut oil, peanut butter, maple syrup, vanilla, and salt until glossy and smooth.

Sift in the cocoa powder to avoid lumps. Stir until fully incorporated.

Add the protein powder in two additions, stirring well each time. The mixture should be thick, fudgy, and spreadable. If it seems too dry, add 1–2 tablespoons of milk (dairy or non-dairy) to loosen. If too loose, add 1–2 tablespoons more protein powder.

Fold in any optional mix-ins. Taste and adjust sweetness or salt if needed.

Scrape the mixture into the prepared pan. Press it into an even layer using a spatula. For a smooth top, lay a piece of parchment over the surface and press with your hands.

Chill in the refrigerator for 1–2 hours, or until firm enough to slice. For faster set, pop it in the freezer for 30–40 minutes.

Lift out using the parchment overhang and transfer to a cutting board. Slice into 16 small squares or 12 larger bars.