Melt butter in a large microwave safe bowl. Whisk the cocoa powder into the butter until smooth. Add 3 eggs and mix until smooth, about 1 minute. Add 3/4 cup granulated sugar and mix for another minute. Add 1/2 cup sour cream and vanilla. Blend until smooth again. Place Coconut flour, baking powder and salt in the bowl and fold into batter until flour disappears. Scrape batter into a lined and greased loaf pan or 8x8 pan and let sit for 10-15 minutes before baking. While batter is sitting in pan, preheat oven to 350 degrees and set rack in center of oven. Place pan in oven and bake for 40-45 minutes for loaf pan (approx 8 1/2 x 4 1/2) or 30 minutes for 8x8 square pan. Cook just until cake is set and toothpick inserted in middle comes out with a few moist crumbs are attached. Remove from oven and let sit for at least 20-30 minutes before removing from pan. Loosen sides of cake by running a butter knife around the edges. Carefully turn over onto a wire rack and let cool completely. When ready to serve, slice and place on plates, serve with ice cream, fresh fruit or whipped cream or dust with powdered sugar.