Coffee Cake with Cinnamon Swirl

Steps

1. Preheat the oven to 175°C (350°F). Lightly grease a 23 × 33 cm (9 × 13 inch) baking pan and line the bottom with parchment paper.
2. Make the cinnamon crumb topping: In a small bowl, combine 70 g all-purpose flour, 60 g light brown sugar, 4 g ground cinnamon, and 1 g fine salt. Add 45 g unsalted butter straight from the refrigerator, cut into small cubes. Rub the butter into the dry ingredients with your fingertips until the mixture forms moist, sandy clumps that hold together when squeezed. Refrigerate while you prepare the cake batter so the crumbs stay firm and bake up crisp.
3. Make the cinnamon swirl: In another small bowl, stir together 40 g light brown sugar, 3 g ground cinnamon, and 5 g all-purpose flour. Melt 15 g unsalted butter in a small saucepan or microwave-safe bowl until fully liquid, then pour it over the sugar mixture and stir until thick and paste-like. Set aside at room temperature; this mixture will be spooned and swirled into the batter.
4. Prepare the dry ingredients for the cake: In a medium bowl, whisk together 250 g all-purpose flour, the baking powder, and the remaining 3 g fine salt until evenly combined and lump-free.
5. In a large mixing bowl, place 110 g unsalted butter at 20–22°C and the granulated sugar. Beat with a hand mixer or stand mixer on medium speed for 3–4 minutes, until the mixture is very pale, creamy, and fluffy with a lighter texture.
6. Add the eggs to the creamed mixture one at a time, beating well after each addition and scraping down the sides of the bowl. The batter should look smooth, slightly thickened, and glossy with no streaks of egg.
7. In a separate bowl or jug, whisk together the sour cream, whole milk, and vanilla extract until smooth and at 20–22°C. This mixture should be pourable and cohesive.
8. Add one-third of the dry ingredients to the butter mixture and mix on low speed just until incorporated. Add half of the sour cream mixture and mix briefly. Repeat with another third of the dry ingredients and the remaining sour cream mixture, finishing with the last third of the dry ingredients. Mix only until the batter is smooth, thick, and pourable with no dry flour pockets. Do not overmix to keep the crumb tender and soft.
9. Assemble the cake: Spread about half of the batter evenly into the prepared pan, creating a level layer that reaches into the corners. Dollop the cinnamon swirl mixture in small spoonfuls over this layer, then use a thin knife or skewer to gently marble it through the batter, creating visible swirls without scraping the pan.
10. Carefully spoon the remaining batter over the swirled layer and spread it into an even layer, completely covering the cinnamon swirl. Break up the chilled crumb topping and scatter it evenly over the surface, pressing very lightly so it adheres without compressing. The top should be fully covered in irregular, chunky crumbs for a crisp, buttery texture after baking.
11. Bake for 30–35 minutes at 175°C (350°F), until the top is golden brown, the crumb topping looks set and crisp, the edges pull slightly away from the sides of the pan, and the center springs back when gently pressed. A toothpick inserted into the center should come out mostly clean or with a few moist crumbs, but no wet batter.
12. Place the pan on a wire rack and let the cake cool in the pan for at least 20–30 minutes until it reaches about 30–35°C, so the crumb sets and slices hold together while still feeling slightly warm to the touch. For the cleanest slices, allow the cake to cool further to room temperature (20–22°C).
13. Cut into squares with a sharp knife. The finished coffee cake should have a soft, moist, tender crumb, a distinct cinnamon sugar ribbon, and a buttery, crisp streusel top that contrasts with the plush interior. Serve at room temperature (20–22°C).