Preheat oven to 325 °F and grease a standard 12-cavity non-stick muffin tin very well (or line with paper liners, if your muffin tin is not very non-stick).
In a large bowl, whisk together the almond flour, sweetener, coconut flour, baking powder, lemon zest, and salt. Stir in eggs, almond milk, butter, and vanilla extract until well combined.
Gently fold in blueberries. Divide batter among prepared muffin cups and bake for 30 to 40 minutes, until set and a tester inserted in the center comes out clean.
Remove and sprinkle with granulated sweetener, if using. Let cool 20 minutes, then transfer to a wire rack to cool completely.
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