Preparation Steps

• Preheat the oven to 350°F. Line a 9x13-inch baking pan with parchment paper leaving a bit of overhang, then lightly grease the paper. This makes it easy to lift the cake out later.
• Make the cinnamon swirl mixture: in a medium bowl whisk together the brown sugar, flour, cinnamon, and nutmeg until evenly combined; set aside.
• Combine the crumble topping: stir the melted butter into the brown sugar, flour, and cinnamon with a fork until it forms coarse crumbs. Press a few clusters together with your fingers for chunkier streusel pieces; refrigerate the crumble while you make the batter so it firms up slightly.
• Prepare the cake batter: in a large bowl cream the room-temperature butter with granulated sugar and brown sugar using a handheld electric mixer (or stand mixer) on medium-high until pale and fluffy, about 2 to 3 minutes.
• Add the eggs one at a time, mixing briefly after each addition, then beat in the vanilla. Scrape down the bowl so everything is even.
• Whisk together the flour, baking powder and salt in a separate bowl. In two additions, add the dry mixture to the butter mixture alternately with the milk, beginning and ending with the dry ingredients. Mix just until combined; overmixing will make the cake dense.
• Fold in the sour cream until smooth and fully incorporated. The batter should be thick but spreadable.
• Assemble the cake in the prepared pan: spread half of the batter into the pan and smooth it to the edges. Sprinkle the cinnamon swirl evenly over that layer, leaving a small border around the pan. Dollop the remaining batter over the swirl and gently spread to cover it, being careful not to press the swirl down.
• Scatter the chilled crumble evenly over the top, pressing a few crumbs lightly so they stay put. Let the assembled pan rest in the fridge for 10 minutes while the oven finishes heating — this helps the streusel hold its shape.
• Bake for 40 to 50 minutes, rotating the pan once halfway through, until the top is golden and a toothpick into the center comes out with a few moist crumbs but no wet batter. If the streusel browns too fast, tent loosely with foil for the last 10 minutes.
• Remove the cake from the oven and set the pan on a wire rack. Let it cool in the pan for about 20 minutes, then lift the cake out using the parchment overhang and cool completely on the rack before glazing.
• Whisk the powdered sugar with 2 tablespoons milk to make a thick but pourable glaze; add a little more milk if needed. Drizzle over the cooled cake with a spoon or a piping bag. Allow the glaze to set 10 minutes before slicing into 12 pieces.