Preheat oven to 350 degrees. In a saucepan, melt 2 sticks of butter over low heat. Increase heat to medium-high and whisk in water and half of cocoa. Bring cocoa-butter mixture to a boil and set aside to cool.

In electric mixing bowl, beat eggs, sugar, baking soda, salt and half of vanilla. While mixer is running, slowly alternate between adding flour and buttermilk and then, finally, the cocoa-butter mixture. Cover a 13½-by-9-by-2-inch pan with cooking spray; pour in batter and bake 20 minutes, or until toothpick comes out clean.

While cake bakes, make frosting by melting remaining butter in a saucepan over low heat. Transfer to mixing bowl; stir in remaining cocoa and milk. Slowly combine powdered sugar and remaining vanilla until smooth. Fold in pecans.

Remove cake from oven and spread frosting evenly over cake while still warm. Makes 8 to 10 servings.