Preheat the oven to 350°F (175°C) and grease a 9-inch cake pan.
In a bowl, mix flour, baking powder, baking soda, and salt.
Dissolve espresso powder in 2 tbsp hot water. Let cool.
Cream butter and sugar until fluffy.
Add eggs one at a time, mixing well.
Stir in sour cream, espresso, and vanilla.
Gradually fold in dry ingredients.
Mix in chocolate chips.
Pour batter into the cake pan.
For the topping, combine flour, cocoa powder, brown sugar, and melted butter. Sprinkle over batter.
Bake for 45 minutes or until a toothpick comes out clean. Let cool.