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1
To make the streusel, combine the brown sugar, walnuts, chocolate, cinnamon, and ground espresso in a small bowl, and stir until well blended.
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2
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).
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3
Line the bottom and sides of a 10-cup loaf pan with foil and grease it lightly (preferably with Pam).
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4
Sift the flour, baking powder, baking soda, and salt together.
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5
Using an electric mixer fitted with the paddle attachment, beat the butter on medium speed for about 3 minutes, until soft and smooth.
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6
Add the granulated sugar and beat on medium-high speed for 5 minutes, until light and fluffy.
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7
Scrape down the sides of the bowl.
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8
Add the eggs, one at a time, and beat until well blended.
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9
Add the vanilla and mix until combined.
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10
Add half of the flour mixture and beat on low speed until just combined.
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11
Add the sour cream and mix until blended, about 1 minute.
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12
Add the remaining flour mixture and mix until just combined.
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13
Do not over mix.
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14
Scrape half of the batter into the prepared pan and spread it evenly.
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15
Evenly distribute half of the streusel mixture over the batter.
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16
Then spoon the remaining batter evenly over the streusel, and spread it evenly.
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17
Scatter the remaining streusel evenly over the top.
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18
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.
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19
Transfer the pan to a wire rack and let cool for 20 minutes.
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20
Lift the cake and the foil liner from the pan, peel away the foil, and return the cake to the rack to cool completely.
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21
Using a serrated knife, cut the coffee cake into 1-inch-thick slices.
