Preheat oven to 350ºF and adjust rack to middle position. Place eggs and coconut sugar in the bowl of a standing mixer. Mix on medium-high for 4-5 minutes, until fluffy and pale yellow. With mixer on medium, slowly add cooled butter, coconut milk, lemon juice and vanilla.
Whisk almond flour, coconut flour, baking powder, baking soda and sea salt in a large mixing bowl. Pour egg mixture over flour mixture and fold until incorporated. Pour batter into a well-oiled (or buttered) bundt pan. Bake for 45-50 minutes until golden brown – when a cake tester is inserted it should come out. Cool for 10 minutes, then invert onto a serving platter. Serve warm or at room temperature.
*To make homemade grain-free baking powder combine 1 part baking soda, 1 part arrowroot powder and 2 parts cream of tartar. Store in an airtight container for up to 6 months.