Sour cream coffee cake with chocolate-cinnamon swirls and hazelnut crumb topping

Ingredients

For the Crumb Topping:

• 3/4 cup all-purpose flour
• 3/4 cup packed light or dark brown sugar
• 1/2 teaspoon salt
• 3/4 cup toasted, skinned hazelnuts
• 6 tablespoons cold butter, cut into small cubes

For the Chocolate-Cinnamon Swirls:

• 1/2 cup sugar
• 1 tablespoon good quality cocoa powder
• 1 teaspoon ground cinnamon

For the Sour Cream Cake:

• 3 1/2 cups all-purpose flour
• 1 teaspoon baking powder
• 1 1/2 teaspoons baking soda
• 1/2 teaspoon salt
• 1 cup (2 sticks) unsalted butter, a little bit softened, but still cool, cut into pieces
• 1 1/4 cups sugar
• 1 cup packed light brown sugar
• 4 large eggs
• 16 ounces sour cream
• 2 teaspoons pure vanilla extract

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Preparation

• Make the Crumb Topping:
  Pulse the flour, sugar and salt together in a food processor to combine. Add the hazelnuts and process until finely chopped and mixed together. Add the butter and pulse a few times to combine. The mixture should look like coarse sand. Place the mixture in a small bowl, cover with plastic wrap and keep the mixture in the refrigerator until ready to use.
• Make the Chocolate-Cinnamon Swirl:
  Whisk the cocoa, sugar and cinnamon together in a small bowl. Set aside.
• Make the Sour Cream Cake:
  Preheat the oven to 350 degrees F. Butter a 9x13-inch baking pan, then line it with a piece of parchment with the edges hanging over the long sides to make a sling. Butter the parchment and set the pan aside.
• In a medium bowl, whisk together the flour, baking powder, baking soda and salt. In the bowl of an electric mixer with paddle attachment, beat the butter until well creamed and smooth. Scrape down the sides of the bowl and add the sugars. Beat the mixture until it is fluffy and light. Add the eggs, one at a time, beating well after adding each egg. Scrape the sides down again and then beat the mixture for 30 seconds more.
• Add the sour cream and vanilla and beat the mixture until just mixed. Add the flour mixture in three parts, scraping down the sides of the bowl in between each addition until just mixed - don't over mix.
• Pour one-third of the cake batter into the bottom of the prepared cake pan and spread it evenly. Sprinkle half of the chocolate mixture over the batter, covering the entire surface of the batter.
• Spoon half of the remaining cake batter over the swirl mixture and spread it evenly. Top the batter with the remaining swirl mixture, covering the surface again. Spread the remaining cake batter evenly over the swirl, then sprinkle the crumb topping over the cake batter.
• Bake the cake in the center of the oven, rotating the pan halfway through baking, for 1 hour or until a skewer comes out clean. Let the cake cool in the pan on a wire rack for 30 minutes before serving.
• Serve from the pan, or use the parchment as a sling to gently lift the cake out onto a cutting surface to cut into squares and serve. The cake keeps well for up to 3 days, tightly covered, at room temperature.