Preparation Steps

• Make the crumb: In a medium bowl, whisk the flour, brown sugar, cinnamon, and nutmeg. Cut in the cold butter with a pastry cutter or your fingertips until the mixture forms moist sandy clumps. Refrigerate the crumb while you prepare the batter.
• Prep the pan and oven: Heat the oven to 350°F (175°C). Grease a 9×13-inch metal baking pan and line it with a parchment sling for easy removal.
• Whisk the dry mix and reserve a pinch for chips: In a bowl, whisk together the flour, baking powder, baking soda, salt, and espresso powder. Remove 1 teaspoon of this mixture to a small dish for tossing the chocolate chips; set both aside.
• Cream the butter and sugar: In a stand mixer fitted with the paddle (or using a hand mixer), beat the room-temperature butter and granulated sugar on medium-high until very light and fluffy, 2 to 3 minutes. Beat in the eggs one at a time, scraping the bowl. Mix in the vanilla.
• Combine the batter: On low speed, add half of the dry mix to the bowl, then all of the sour cream, then the remaining dry mix. Mix just until the flour disappears. Let the batter stand 5 minutes to thicken slightly.
• Fold in the chips: Toss the mini chocolate chips with the reserved teaspoon of dry mix, then fold them into the batter with a spatula until evenly distributed.
• Assemble: Spread a little over half of the batter into the prepared pan. Sprinkle about two-thirds of the chilled cinnamon crumb evenly over the batter. Dollop the remaining batter on top and gently spread to cover. Finish with the remaining crumb.
• Bake: Place on the center rack and bake 40 to 48 minutes, rotating the pan once halfway through, until the top is deeply golden and a toothpick inserted in the center comes out clean or with a few moist crumbs.
• Cool and slice: Let the cake cool in the pan for 20 minutes, then lift it out using the parchment and cool completely before slicing. Tip: For extra-neat squares, chill the cooled cake for 30 minutes before cutting.