01 -
In a medium bowl, mix together the cold, cubed butter, 3/4 cup light brown sugar, 1/2 cup flour, 1/4 teaspoon salt, and 1/4 teaspoon cinnamon. Use a fork or pastry cutter to blend until the mixture resembles coarse crumbs. Stir in the chopped walnuts or pecans and 1 cup chocolate chips. Set aside.
02 -
In a large bowl, whisk together 2 cups flour, 1/2 cup light brown sugar, 1/4 cup granulated sugar, baking powder, baking soda, 1/2 teaspoon salt, and optional espresso powder.
03 -
In a separate bowl, whisk together the sour cream, oil, milk, eggs, and vanilla extract until smooth and creamy.
04 -
Pour the wet ingredients into the dry ingredients and gently fold together with a spatula or wooden spoon just until no streaks of flour remain. Be careful not to overmix.
05 -
Gently fold in 3/4 cup chocolate chips. The batter will be thick.
06 -
Preheat oven to 350°F (175°C). Grease a 9x9-inch pan. Spread half the batter evenly into the pan, sprinkle half the streusel topping over it, then spoon the remaining batter on top, gently spreading it. Top with the rest of the streusel.
07 -
Bake for 40–45 minutes or until a toothpick inserted into the center comes out mostly clean (a few melty chocolate streaks are fine). If the top is browning too fast, loosely tent with foil.
08 -
Allow the cake to cool in the pan for 15–20 minutes. Cut into squares, serve warm or at room temperature, and enjoy!