Preheat oven to 350°F (175°C). Grease and line a 9×9-inch (23cm) square baking pan with parchment paper.

Make the crumb topping: Whisk together flour, both sugars, lemon zest, cinnamon, ginger, and salt. Pour in melted butter and stir with a fork until large, shaggy clumps form and no dry flour remains. Refrigerate while you prepare the batter.

Make the batter: Whisk flour, baking powder, and salt in a bowl; set aside. In a stand mixer fitted with the paddle attachment, beat butter and sugar on medium-high until pale and fluffy, about 3 minutes. Add eggs one at a time, beating well after each. Mix in vanilla, lemon zest, and lemon juice (if using). Reduce speed to low and alternate adding the flour mixture and sour cream in three additions, beginning and ending with flour. Mix until just combined — do not overmix.

Assemble: The batter will be thick; use an offset spatula to spread it evenly into the prepared pan. If your lemon curd is quite loose, chill it for 20–30 minutes before using. Dollop curd over the surface and gently swirl with a skewer or knife — you want visible ribbons, not full incorporation. Scatter the chilled crumb topping evenly over the top, pressing lightly to adhere.

Bake until the centre is set, the crumb is deep golden, and a toothpick inserted into the cake (not the curd) comes out with moist crumbs and no streaks of wet batter. This typically takes 40–45 minutes. If the top browns too quickly, tent loosely with foil after 30 minutes.

Cool in the pan for at least 20 minutes (longer for cleaner slices) before lifting out. Dust generously with powdered sugar just before serving. Best served slightly warm or at room temperature.

Optional: For a brighter finish, drizzle with a simple lemon glaze instead of dusting with powdered sugar.