Mix the flour, granulated sugar, and baking soda in a bowl. Slowly add the cooled butter and cocoa mixture to the dry ingredients, stirring until well combined. Mix in the sour cream and eggs to the batter until smooth.
Transfer the batter to the prepared baking pan and spread it into a smooth, even layer. Bake for 20-25 minutes or until a toothpick inserted in the center comes out clean.
While the cake is baking, make the frosting by melting the butter in a saucepan. Stir in the cocoa powder and milk. Remove from the heat and allow it to cool slightly. Whisk in the confectioner’s sugar until smooth.
Once the cake is baked and slightly cooled, pour the frosting over the warm cake and spread evenly with a spatula. Sprinkle the top with the chopped pecans for decoration.