Combine 2 tablespoons butter and brown sugar until combined. Add 2 tablespoons flour, cinnamon, and pecans, and mix until combined.

Sprinkle the mixture evenly in the bottom of the prepared pan. Set aside while you make the cake batter.

Whisk together the flour, baking powder, baking soda, and salt. Set aside.

Using an electric mixer on medium speed, beat the butter and sugar until light and fluffy. Add the eggs, one at a time, mixing well after each addition. Mix in the vanilla and orange zest.

Reduce mixer speed to low. Add the flour mixture in three portions, alternating with two portions of sour cream. Mix just until combined. Stir in the chocolate chunks.

Transfer the batter to the prepared pan, and spread evenly.

Bake 65 to 75 minutes, or until the cake is browned and a pick inserted into the center comes out clean.

Cool in the pan on a wire rack for 10 minutes. Then transfer the cake directly to a wire rack to cool completely.