1. Preheat oven to 350°F (175°C). Line an 8-inch (20cm) square baking pan with parchment paper.

2. Prepare the crumb topping: In a bowl, mix flour, brown sugar, cinnamon, and salt. Cut in cold butter until the mixture resembles pea-sized crumbs. Chill until ready to use.

3. In a large bowl, cream softened butter and granulated sugar until light and fluffy.

4. Add eggs one at a time, beating well after each addition.

5. Mix in sour cream, vanilla extract, and espresso powder until combined.

6. In a separate bowl, whisk together flour, baking powder, baking soda, and salt.

7. Gradually add dry ingredients to the wet mixture, mixing until just combined.

8. Transfer half of the batter to another bowl and stir in cocoa powder to create the chocolate batter.

9. Alternate spoonfuls of vanilla and chocolate batters into the prepared pan. Use a knife to swirl the batters together gently.

10. Sprinkle the chilled crumb topping evenly over the batter.

11. Bake for 35–38 minutes, or until a toothpick inserted into the center comes out clean.

12. Allow the cake to cool in the pan for 15 minutes before transferring to a wire rack to cool completely.