Stir together allulose, stevia, peanut butter, collagen (or cream powder), butter, and almond milk in medium saucepan over medium heat. Continue to cook and stir until the butter has melted. Remove from heat and stir in vanilla extract if using.
Quickly transfer to a wax paper lined 8×8 dish. The fudge will begin to harden quickly. When cooled completely, cut into slices. Keep refrigerated in airtight container.