Preheat the Oven & Prepare the Crumb Topping:

• Preheat the oven to 350°F (175°C). Grease and flour an 8×8 square baking pan.

• In a mixing bowl, combine flour, brown sugar, cinnamon, and salt. Add the cold butter cubes and use a pastry cutter or your fingers to combine until the mixture resembles coarse crumbs. Set aside.

Prepare the Cake Batter:

• In a stand mixer, cream together the granulated sugar and unsalted butter until light and fluffy. Add the eggs one at a time, mixing well after each addition.

• Add the sour cream and vanilla extract, and mix until combined.

• In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing just until combined.

• Divide the batter in half. To one half, mix in the instant espresso powder. To the other half, add the cocoa powder and chopped dark chocolate, stirring until well combined.

Assemble the Marble Cake:

• Drop spoonfuls of the espresso batter and chocolate batter alternately into the prepared pan. Use a large spoon or knife to swirl the two batters together to create a marbled effect.

• Sprinkle the crumb topping evenly over the top of the cake batter.

Bake:

• Bake the cake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center comes out clean.

• Let the cake cool in the pan for about 15 minutes before transferring to a wire rack to cool completely.