Instructions

• Preheat the oven to 350°F (175°C). Butter an 8x8-inch square baking pan, line it with parchment paper leaving an overhang for easy removal, and lightly butter the parchment.
• Make the topping: in a microwave-safe bowl, combine 6 tablespoons (85 g) unsalted butter, melted, with 1/4 cup (50 g) granulated sugar and 1/4 cup (55 g) packed light brown sugar. Stir until evenly moistened.
• Add 1 cup (125 g) all-purpose flour, 4 teaspoons instant espresso powder, 1/4 teaspoon ground cinnamon, and a pinch of fine sea salt to the butter–sugar mixture. Mix until a crumbly dough forms that holds together when squeezed. Set the topping aside to cool.
• In the bowl of a stand mixer fitted with the paddle attachment or in a large mixing bowl with a hand mixer, cream 1/4 cup (56 g / 1/2 stick) unsalted butter (room temperature) and 1/2 cup (100 g) granulated sugar together until light and fluffy, about 2–3 minutes.
• Add 1 large egg (room temperature) and 2 teaspoons vanilla extract to the creamed butter and sugar. Mix until incorporated.
• In a small bowl, whisk 1/2 teaspoon baking soda into 1/2 cup (130 g) full-fat sour cream until smooth.
• In a separate bowl, whisk together 1 cup (125 g) all-purpose flour, 3/4 teaspoon baking powder, and 1/2 teaspoon fine sea salt.
• Add half of the dry ingredients to the butter mixture and mix just until combined. Add half of the sour cream mixture and mix until combined. Scrape down the sides of the bowl, then add the remaining dry ingredients and then the remaining sour cream mixture, mixing each addition just until incorporated. Do not overmix.
• Spread the batter evenly into the prepared pan and smooth the top with a spatula.
• Crumble the cooled topping evenly over the batter, starting at the outer edges and working toward the center so the entire top is covered. Press lightly so some crumbs adhere but keep some larger pieces for texture.
• Bake for 30–35 minutes, or until a toothpick inserted near the center comes out with a few moist crumbs attached. Remove from the oven and place the pan on a wire rack to cool completely.
• Make the glaze: sift 1 cup (120 g) confectioners' sugar into a bowl. Add 2 tablespoons freshly brewed espresso (or 1 tablespoon instant espresso powder dissolved in 2 tablespoons hot water) and a pinch of fine sea salt. Whisk until smooth. The glaze should be thick but still drizzle-able. If it is too thin, add additional sifted confectioners' sugar 1 tablespoon at a time to thicken. If it is too thick, add additional brewed espresso 1/2 teaspoon at a time to thin.
• When the cake is completely cool, use a whisk or fork to drizzle the glaze evenly over the top. Let the glaze set about 15 minutes before using the parchment overhang to lift the cake from the pan and cutting into squares.
• Store leftover cake covered in an airtight container for up to 3 days.