• Preheat the oven to 350F (180C) degrees.
• Wrap the outside of a 9-inch springform pan with aluminum foil. Lightly grease the pan and dust with flour. Set aside.

Crumb Topping

• Stir together the flour, granulated sugar, brown sugar and salt
• Stir in the melted butter. It should be like crumbly, wet sand. If it feels too wet, mix in 1 extra tablespoon of flour and granulated sugar. Set aside.

Lemon Cake

• In a medium bowl sift together the flour, baking powder, baking soda, and salt. Then whisk the ingredients together. Set aside.
• Add the sugar and lemon zest to a large bowl. Using your fingers, pinch/rub the lemon zest into the sugar. This activates the lemon flavor.
• Add the butter to the sugar bowl. Beat the butter and sugar mixture together until fluffy.
• Add in the vanilla extract. Then beat in the eggs one at a time, turning off the mixer and scraping down the sides and bottom of the bowl as you go.
• Beat the sour cream and lemon juice into the butter mixture.
• Add about ½ of the flour mixture into the butter mixture. With the mixer on a low speed, beat together until just combined.
• Whisk the milk into the batter. Alternatively, use an electric mixer on a low speed.
• Add the rest of the flour mixture into the batter. Gently mix together until you no longer see streaks of flour. Do not over mix. I typically fold in the flour by hand, then if needed, beat for a few seconds with the electric mixer on a low speed.

Assembling and Baking

• Spoon/pour the batter into the prepared pan and smooth the top.
• Sprinkle the crumb topping over top of the batter, so that the the cake is fully covered in an even layer of crumb topping. It will look like a lot.
• Bake in the middle of the preheated oven for about 45-50 minutes if using a 9-inch round springform pan or about 35 minutes if using a 9x9 inch square pan. When the cake is done baking, an inserted toothpick should come out clean and the cake shouldn't wobble if you give the pan a nudge. If you notice that the crumble is starting to brown, but the cake isn't close to being done yet, tent a piece of aluminum foil over the top.
• Cool the cake in the pan.

Lemon Glaze

• Whisk together 1 tablespoon fresh lemon juice with about 1 cup of powdered sugar.
• Slowly whisk in a little more lemon juice or powdered sugar as needed. You want the glaze to be thin enough to drizzle but still opaque.
• Unclamp the outer ring of the springform pan and drizzle the cooled cake with glaze.