• Preheat the oven to 350°F. Line an 8-inch square metal pan with parchment paper or foil and spray with nonstick cooking spray.

Make the streusel:

• In a medium microwave-safe bowl, place the butter and microwave until melted. Allow to cool until just warm (about 80°F). Add in the sugar, flour, salt, and cinnamon with a fork until thick. Mix in the nuts and chocolate chips. Mixture will be crumbly. Set aside.

Make the cake:

• In a large bowl, whisk together the flour, brown sugar, granulated sugar, baking powder, baking soda, salt, and espresso powder (if using). In a medium bowl, whisk together the sour cream, oil, milk, eggs, and vanilla until well combined. Make a well in the center of the dry ingredients, then pour in the wet ingredients. Gently stir until a few streaks of flour remain. Gently stir in the chocolate chips.
• Spoon just less than half of the batter into the prepared pan. Sprinkle with half of the streusel. Spread the remaining batter over the streusel. Sprinkle with the remaining streusel, gently pressing down into the batter.
• Bake about 48 minutes, or until the edges are lightly golden brown and the center is cooked through or a digital thermometer inserted into the center reads about 190°F. Cool until just warm or at room temperature before slicing and serving.
• Store inside an airtight container at room temperature for up to 2 days, or in the fridge for up to 3 days.