Preheat the oven to 350°F and line a muffin pan with paper liners.
Combine ½ cup butter, ¾ cup granulated erythritol, 1 ½ cups almond flour, and ½ cup coconut flour with the sour cream, lemon juice, baking powder, vanilla extract, and xanthan gum in a blender.
Blend until smooth and well combined, scraping down the sides as needed.
Melt the remaining butter then stir in the remaining almond flour, granulated erythritol, and coconut flour.
Stir in the lemon zest until it forms a crumbly mixture.
Divide the muffin batter among the 12 cups and top each with lemon streusel topping.
Bake for 30 to 35 minutes until a knife inserted in the center comes out clean.