Preheat the oven to 350ºF and grease 6 wells of a standard metal muffin pan very well (butter or coconut oil works best).
In a medium microwave safe bowl, melt the butter. Stir in the two sweeteners until well combined, then spoon into the bottoms of the prepared muffin cups.
Arrange the peach slices over the sweetener mixture and set aside.
Cake
In a medium bowl, whisk together the almond flour, sweetener, protein powder, baking powder, cinnamon, and salt. Add the egg, melted butter, and vanilla and stir until well combined.
Add just enough liquid to make a thick but scoopable batter. Divide among the muffin cups, filling almost to the top. (Don't overfill... if you have leftover batter, make an extra plain muffin with it!).
Bake 18 to 22 minutes, until the tops are puffed and firm to the touch. Remove and let cool 5 minutes, then take a knife and loosen around the edges of each cake.
Flip out onto a wire rack and cool another 15 minutes or so. Serve warm or room temperature.