Preheat the oven to 190C/375F and line a 12-hole muffin tin.

In a bowl, combine buckwheat flour, tapioca flour, potato starch, xanthan gum, and baking powder.

In a separate bowl, mix lemon juice, zest, vanilla extract, oil, coconut milk, granulated sugar, and brown sugar until the sugars dissolve.

Pour the liquid mixture into the dry mixture and stir until just combined.

Add the chia seeds and stir until evenly distributed.

Fill the muffin cups 3/4 full with the batter.

Bake for 25 minutes or until a toothpick inserted into the center comes out clean.

Let the cupcakes cool in the tin for a few minutes before transferring them to a wire rack to cool completely.