01 - Preheat the oven to 350°F. Grease and flour a 9-inch round or square baking pan.
02 - In a small bowl, mix flour, sugar, lemon zest, and salt. Cut in the cold butter with a fork or pastry blender until the mixture resembles coarse crumbs. Refrigerate until ready to use.
03 - In a large bowl, cream the softened butter and sugar until light and fluffy. Beat in the eggs one at a time, then add the vanilla extract, lemon zest, and lemon juice.
04 - In a separate bowl, whisk together flour, baking powder, baking soda, and salt.
05 - Alternately add the dry ingredients and milk to the butter mixture, beginning and ending with the dry ingredients. Mix until just combined; do not overmix.
06 - Pour the batter into the prepared pan and smooth the top. Evenly sprinkle the crumble topping over the batter.
07 - Bake for 38-42 minutes, or until a toothpick inserted into the center comes out clean and the crumble is golden brown.
08 - Let the cake cool in the pan for 15 minutes, then transfer to a wire rack to cool completely. For the optional glaze, whisk powdered sugar with lemon juice until smooth and drizzle over the cooled cake.