Preheat oven to 350°F. Line a cupcake pan with 12 paper liners, or lightly grease and (brown rice) flour the individual cups.
In a large mixing bowl, whisk together the brown rice flour, sweet white rice flour, potato starch, besan, xanthan gum, baking powder, baking soda, and salt.
In the bowl of electric mixer, cream together the brown sugar, olive oil, and coconut milk. Gently add in the flour mixture, alternating with the water. Add the lemon juice and mix on high speed for about 1 to 2 minutes. Divide batter among the 12 muffin tins and bake for 35 minutes, or until puffed up high and golden brown, and knife inserted into center comes out clean. Let the cupcakes cool completely before filling and topping.
To make the filling, combine all the ingredients using an electric mixer with a whisk attachment and whip until fluffy. Using a serrated knife, cut off tops of cupcakes just above the papers. Add about 2 tablespoons of filling and replace the top. Place cupcakes in the freezer on a flat surface a few minutes before topping with Dark Chocolate Ganache.
Store loosely covered cupcakes in the refrigerator for up to 1 week.