Preheat oven to 350ªF. Grease and flour 3-9" round layer pans.
Stir together cocoa and water in a small bowl until smooth; set aside.
Beat butter, sugar and vanilla in a stand mixer until light and fluffy, scraping down sides. Add eggs, one at a time, beating well after each addition.
Stir together flour, baking soda and salt. Add to butter mixture alternating with chocolate mixture and buttermilk, beating only enough to blend. Pour into prepared pans.
Bake 22-28 minutes. Cool 5 minutes, then turn out on to wire rack to cool completely. Prepare Coconut Pecan Frosting; spread between first and second layer.