Heat oven to 350 degrees. Lightly butter an 8-by-4-inch loaf pan. Melt coconut oil in small saucepan over low heat. Bring oil back to room temperature before baking. In a medium mixing bowl, combine rice flour, baking powder, baking soda, salt and pepper, if using.

Using an electric mixer, whisk together coconut oil, butter and sugar at high speed until light and fluffy, 2 to 3 minutes. With the motor still running, add eggs and combine until creamed and very fluffy. Reduce to low speed and whisk in a third of the flour mixture. Once combined, add sour cream. Whisk in another third of the dry ingredients, then half of the coconut milk. Add the remaining flour mixture. At a medium speed, whisk in the rest of the coconut milk, and mezcal, until smooth and completely combined.

Scrape batter into pan and smooth the top with the spatula. Bake until the cake is golden and a toothpick inserted into the center comes out clean, about 45 to 55 minutes. Let cake cool in pan for 10 minutes, then turn on a wire rack to cool completely.