To make the streusel, combine the brown sugar, walnuts, chocolate, cinnamon, and ground espresso in a small bowl, and stir until well blended.

To make the cake, position an oven rack in the middle of the oven, and preheat the oven to 325 degrees F (325 degrees F if using a convection oven). Line the bottom and sides of a 10-cup loaf pan with foil and grease it lightly (preferably with Pam).

Sift the flour, baking powder, baking soda, and salt together.

Using an electric mixer fitted with the paddle attachment, beat the butter on medium speed for about 3 minutes, until soft and smooth. Add the granulated sugar and beat on medium-high speed for 5 minutes, until light and fluffy. Scrape down the sides of the bowl. Add the eggs, one at a time, and beat until well blended. Add the vanilla and mix until combined. Add half of the flour mixture and beat on low speed until just combined. Add the sour cream and mix until blended, about 1 minute. Add the remaining flour mixture and mix until just combined. Do not over mix.

Scrape half of the batter into the prepared pan and spread it evenly. Evenly distribute half of the streusel mixture over the batter. Then spoon the remaining batter evenly over the streusel, and spread it evenly. Scatter the remaining streusel evenly over the top.

Bake for 68 to 70 minutes (50 to 60 minutes if using a convection oven), until the topping is browned and a toothpick inserted in the center of the cake comes out clean.

Transfer the pan to a wire rack and let cool for 20 minutes. Lift the cake and the foil liner from the pan, peel away the foil, and return the cake to the rack to cool completely. Using a serrated knife, cut the coffee cake into 1-inch-thick slices.