Preheat the oven to 350 degrees.

In a large bowl cream together the butter, sugar, and brown sugar.

Add in the eggs one at a time, beating after each addition.

In a small bowl combine the almond, hazelnut, and brown rice flour. Add in the baking powder and mix well.

Add the flour mixture to the sugar mixture and beat until combined.

Whisk in the mashed potatoes until smooth. Stir in the salt, lemon juice, and lemon zest.

Pour the mixture into a greased 9 inch cake pan.

Bake for 40-45 minutes or until a toothpick inserted into the center comes out clean.

Remove from the oven and cool for 1 hour on a wire rack.

In a small bowl whisk together the powdered sugar and lemon juice. Drizzle over top of the cooled cake.