Fit waxed paper into an 8x4-inch or 5x9-inch loaf pan, leaving enough paper above the sides to use as handles. Press the paper into the corners so the fudge will set in an even layer.

Warm the peanut butter and butter together in a medium saucepan over low heat, stirring until both melt into one mixture. Keep the heat quiet and gentle; sizzling is a sign to lower it immediately.

Stir the vanilla extract into the melted peanut butter mixture, then whisk in the powdered sweetener and collagen peptides until smooth. The finished mixture should flow in a glossy ribbon without oily streaks or dry powder.

Scrape the fudge into the lined pan and refrigerate until set, about 1 hour. The slab should feel firm when pressed lightly before you lift it out by the paper edges and cut it into 15 pieces.