01 - Preheat oven to 350°F. Grease and flour a 9-inch round or square baking pan.
02 - In a small bowl, combine flour, sugar, lemon zest, and salt. Cut in cold butter with a fork or pastry blender until mixture resembles coarse crumbs. Refrigerate until ready to use.
03 - In a large bowl, cream softened butter and sugar until light and fluffy. Beat in eggs one at a time, then add vanilla extract, lemon zest, and lemon juice.
04 - In a separate bowl, whisk together flour, baking powder, baking soda, and salt.
05 - Alternately add dry ingredient mixture and milk to butter mixture, beginning and ending with dry ingredients. Mix until just combined without overmixing.
06 - Pour batter into prepared pan and smooth the top evenly.
07 - Evenly sprinkle crumble topping over batter.
08 - Bake for 38 to 42 minutes, or until a toothpick inserted into the center comes out clean and the crumble is golden brown.
09 - Let cake cool in pan for 15 minutes, then transfer to a wire rack to cool completely.
10 - Whisk powdered sugar with lemon juice until smooth and drizzle over cooled cake.