Preheat oven to 350°F. Line an 8x8-inch square baking pan with parchment paper leaving a 2-inch overhang at each side.

Make the crumb topping. Combine flour, both sugars and salt (if you’re using unsalted butter) in a medium bowl and whisk to blend evenly so that there are no lumps of brown sugar. Add melted butter to the dry ingredients and stir with a spoon until evenly combined, then toss with your fingertips to form large crumbs. Cover with a tea towel and set aside.

Make the cake batter. Combine melted butter, sugar, vanilla and lemon zest in a medium mixing bowl and whisk until pale and fluffy-looking. Whisk in eggs one at a time until very light and creamy. Mix in sour cream. Sift flour, baking powder and salt into a small bowl and whisk to blend evenly. Add it to the wet mixture and fold it in by hand with a spatula until mostly incorporated. Pour in milk and fold it in until batter is evenly combined.

Spread batter evenly into prepared pan. Dollop spoonfuls of lemon curd over the batter and swirl it in with a knife. Then, scatter clumps of crumb topping evenly on top to cover the lemon curd. Make sure the the crumbs cover most of the lemon curd because exposed curd can burn in the oven if you have hot spots.

Bake for 30-35 minutes until the crumbs are lightly golden and the cake feels firm to the touch. A toothpick inserted into the center should comes out without any wet batter attached. Transfer to a wire rack and let cool for at least 30 minutes.

Make the glaze. Combine powdered sugar, lemon juice and salt in a small bowl and whisk until smooth. Drizzle over the cooled cake and enjoy!