Heat the oven to 350°F (175°C). Grease an 8-inch or 9-inch square baking pan and line it with parchment paper, leaving a little overhang for lifting the cooled cake.

Make the crumb topping by whisking together the flour, sugar, lemon zest, and salt. Stir in the melted, slightly cooled butter until large, irregular crumbs form, then set the mixture aside.

Whisk the cake flour, baking powder, baking soda, and salt in a separate bowl. In another bowl, beat the softened butter and granulated sugar for 2–3 minutes, until pale and fluffy.

Beat in the eggs one at a time, making sure each is incorporated before adding the next. Mix in the lemon zest, fresh lemon juice, and vanilla extract.

Add the dry ingredients in two additions, alternating with the sour cream or Greek yogurt. Mix only until combined, taking care not to overmix the batter.

Spread the batter evenly in the prepared pan. Scatter the lemon crumb mixture over the surface, pressing a few pieces lightly into the batter.

Bake for 35–42 minutes, until the center is set and a toothpick inserted into the cake portion comes out clean or with a few moist crumbs. An 8-inch pan usually needs the longer end of the baking range, while a 9-inch pan may finish several minutes sooner.

Cool the cake in the pan for at least 20 minutes. For the glaze, whisk the powdered sugar with 1 1/2 to 2 tablespoons fresh lemon juice, then drizzle it over the cooled cake before slicing.