Heat the oven to 350°F (175°C). Grease a 9-inch square baking pan and line it with parchment, leaving enough overhang to lift out the cooled cake.

Make the crumble by rubbing or cutting the butter into the flour and sugar until coarse crumbs form. Stir in the optional lemon zest, then refrigerate the topping while preparing the batter.

Whisk the flour, baking powder, and salt together in a medium bowl. In a separate large bowl, cream the softened butter and sugar for 2–3 minutes, until light and fluffy.

Beat in the eggs one at a time, mixing after each addition. Stir in the vanilla, finely grated lemon zest, and fresh lemon juice.

Add the dry ingredients in two or three additions, alternating with the sour cream and milk if needed. Mix only until the ingredients are combined; the batter should be fairly thick.

Spread the batter evenly into the prepared pan. Sprinkle the chilled crumble over the surface, covering the cake from edge to edge.

Bake for 40–50 minutes, checking early if using a dark metal pan. The center should be set, and a toothpick inserted in the middle should come out with only a few moist crumbs.

Cool the cake in the pan for at least 15 minutes before glazing or slicing. For the glaze, whisk the powdered sugar with lemon juice, adding the juice gradually until thick but pourable, then drizzle over the cake.