In a medium microwave-safe bowl, melt the peanut butter and butter on high in 30-second increments. Whisk in the sweetener and vanilla extract, then whisk in the egg until smooth.
Add the collagen, cocoa powder, and salt, stirring until well combined. Stir in just enough water to thin the batter until it is easily pourable.
Divide the mixture between 6 silicone muffin cups. Press one peanut butter cup into the cups until just covered by the brownie batter.
Preheat the air fryer to 300ºF. Air fry the brownies until they are puffed and just firm to the touch on top, about 7 to 10 minutes.
Remove and let cool before serving.
Notes
Nutrition information is an estimate, calculated automatically. Values vary with brands, substitutions and portion sizes.