Step 1In a large bowl, stir together the brown sugar, cocoa, cinnamon, chocolate chips, pecans and butter. Set aside.
Step 1Heat the oven to 350 degrees.
Step 2In the bowl of a stand mixer, or in a large bowl using a hand mixer, cream together the butter and sugar until light and fluffy, about 3 to 5 minutes. Beat in the eggs, one at a time, until incorporated, then beat in the sour cream and vanilla.
Step 3In a medium bowl, sift together the flour, baking soda and salt.
Step 4Gently fold the dry ingredients into the butter and egg mixture to form a batter. The batter will be lumpy; simply continue folding until no large lumps remain.
Step 5Spoon one-half of the batter into a greased 9-inch-square baking pan, then layer with half of the topping. Layer with the remaining batter, then the remaining topping. Bake until the coffee cake is puffed and golden and a toothpick inserted in the center comes out clean, about 45 minutes. Cool slightly before serving.