Preheat oven to 350°F. Grease and flour 2 (10-inch by 3-inch-deep cake pans. Cream butter until smooth, add brown sugar and eggs, beat on high speed until light and fluffy. Add vanilla, cocoa, baking soda and salt on low speed. Add flour and sour cream alternately to mixture on low speed until smooth. Pour in boiling water; stir with spoon until well blended. Divide mixture evenly between the pans. Bake 35 minutes or until done. Cool 10 minutes, loosen onto wire racks. Cool. 

For filling and frosting: Preheat oven to 350°. Bake pecans in a single layer in a shallow pan for 8–10 minutes, or until toasted and fragrant, stirring halfway through. Cool completely (about 20 minutes). 

Meanwhile, cook the evaporated milk, sugar, butter and egg yolks in a heavy 3-quart saucepan over a medium heat, stirring constantly, 3–4 minutes, or until butter melts and sugar dissolves. Cook, stirring constantly, 12–14 minutes, or until mixture becomes a light caramel color, is bubbling and reaches a pudding-like thickness. Remove pan from heat; stir in coconut, vanilla and pecans. Transfer mixture to a bowl. Let stand, stirring occasionally, 45 minutes or until cooled and spreading consistency. 

Spread layer of coconut and pecan frosting on bottom layer cake. Add second layer cake and ice the entire cake.