In a large mixing bowl, combine coconut oil, coconut sugar, and cocoa powder and stir until combined. Add eggs and vanilla next, stirring until just blended. Do not over-mix.
In a separate bowl, combine almond flour, coconut flour, baking powder, and baking soda. Add a portion of the dry ingredients to the brownie mixture and stir until combined. Then add a little of the milk to the mixture and stir. Repeat this process, alternating between adding dry ingredients and the milk until both are gone.
Pour mixture into a greased 9×13 baking pan and bake for approximately 18-23 minutes, or until an inserted toothpick comes out clean.
Paleo baking is a little different than traditional baking, so I typically check my brownies at around the 15-minute mark, and then periodically thereafter until it reaches the desired consistency.