• Preheat oven to 350°F (177°C). Line a 9×13-inch baking dish with parchment paper, leaving overhang for easy removal. Then, spray with nonstick cooking spray. Set aside.

Crumb Topping

• In a medium bowl, whisk together flour, granulated sugar, light brown sugar, lemon zest, and salt.
• Add the cold cubed butter, and use a pastry cutter or your fingertips to work the butter into the dry mixture until large, sandy crumbs form, with some pea-sized pieces remaining.
• Place the crumb topping in the refrigerator while you make the cake batter.

Cake

• In the bowl of a stand mixer with the paddle attachment, cream together butter and granulated sugar until light and fluffy. Add eggs one at a time, mixing well after each.
• Mix in lemon zest, lemon juice, and vanilla.
• In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. Alternate adding the dry ingredients and sour cream in three additions (dry, sour cream, dry, sour cream, dry), beginning and ending with dry, and mix on low just until combined.
• Mix in the milk just until smooth, being careful not to overmix.

Assembly

• Spread half of the cake batter evenly into the prepared pan. Dollop spoonfuls of the chilled homemade lemon curd over the batter. Gently swirl the curd with a knife or skewer, leaving visible streaks rather than fully blending it in.
• Spread the remaining batter on top, smoothing gently but completely.
• Sprinkle the chilled crumb topping evenly over the cake. Press very lightly so the crumbs stick, but still maintain a chunky appearance.
• Bake for 40 to 50 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs (avoid hitting the lemon curd swirl). Rotate the pan halfway through baking for even browning. If the crumbs are browning too quickly, tent loosely with foil for the last 10 minutes.
• While the cake bakes or cools, whisk together the confectioners’ sugar and lemon juice in a small bowl until smooth and pourable. Adjust with more lemon juice if needed for desired consistency.
• Let the cake cool in the pan for at least 20 to 30 minutes before slicing. Drizzle with glaze if using, and serve warm or at room temperature.