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

Mix the dry: In a large bowl, whisk the protein powder, cocoa, and salt until no lumps remain.

Stir the wet: In a separate bowl, combine nut butter, applesauce, vanilla, and sweetener. Stir until smooth. If your nut butter is firm, microwave it for 10–15 seconds to loosen.

Combine: Add the wet mixture to the dry. Stir while slowly adding milk, starting with 1/3 cup. You want a thick, scoopable batter—denser than brownie batter but not crumbly. Add a splash more milk if needed.

Fold in extras: If using chocolate chips or nuts, fold them in now.

Press and smooth: Transfer the mixture to the prepared pan. Press it evenly into the corners with a spatula. Smooth the top.

Chill: Refrigerate for at least 2 hours, or freeze for 30–40 minutes, until firm enough to slice.

Slice and serve: Lift the slab out with the parchment and cut into 16 squares. Sprinkle with flaky sea salt if you want a sweet-salty finish.