01 - Preheat oven to 350°F and grease and flour a 9-inch round or square cake pan.
02 - Combine flour, sugar, cinnamon (if using), and salt in a small bowl. Cut in cold butter using a fork or fingers until coarse crumbs form. Refrigerate.
03 - Beat butter and sugar together until light and fluffy. Add eggs one at a time, mixing well after each. Stir in lemon zest, lemon juice, sour cream, milk, and vanilla extract.
04 - Whisk flour, baking powder, baking soda, and salt separately. Gradually add to wet mixture, mixing until just combined.
05 - Pour batter into prepared pan and spread evenly. Evenly sprinkle crumble topping over the surface.
06 - Bake 38 to 42 minutes until a toothpick inserted at the center comes out clean.
07 - Cool cake in pan for 10 minutes, then transfer to a wire rack to cool completely.
08 - Whisk powdered sugar and lemon juice until smooth. Drizzle over cooled cake before serving.