Preheat oven to 325 degrees. Grease an 8x4 inch loaf pan with butter or melted coconut oil. My personal preference is lining the pan with parchment paper for easier removal!
In large bowl, combine butter and sugar and beat until smooth. Add the eggs and beat until well blended. Add lemon juice, lemon zest, and vanilla and mix again until batter is smooth.
In separate bowl, combine dry ingredients: almond flour, coconut flour, baking powder, and sea salt.
Add dry ingredients to butter mixture and beat to combine.
Add coconut milk and beat again until batter is very smooth.
Pour batter into greased loaf pan and bake for about 60-70 minutes. Toothpick inserted in center should come out smooth, and sides should begin to pull away from sides. Top will be very golden.
Let cool for about 30 minutes, preferably longer. Loosen loaf around sides and flip onto cooking rack or serving tray to cool completely. Or lift out of pan by holding onto parchment paper. (You can even refrigerate it for a while to make sure it's set! Or you can ice it while still in the pan.)