Step 1

Combine the sugar and lemon zest in a small food processor. Pulse or process until the zest is incorporated into the sugar and the sugar has turned a pale yellow color.

Step 2

Melt 4 tablespoons of butter in a small bowl. Add the flour, lemon sugar and lemon juice.  Mix with a fork to combine.  Set aside.

Step 3

In the same small food processor used above, add half the granulated sugar and the zest from two lemons. Process until combined and the sugar is a light yellow color.

Step 4

Preheat oven to 325°F. Invert the bottom of an 8" springform pan (to make it easier to slide the cake off the bottom) then lock on the sides of the pan. Lightly grease and flour the bottom and sides of the pan. Line the bottom with a round of parchment paper and set aside.

Step 5

In a small bowl combine the flour, salt and baking powder. Set aside.

Step 6

In the bowl of a stand mixer fitted with the paddle attachment, combine the butter, remaining granulated sugar and the lemon sugar. Beat for several minutes until light and fluffy. Scrape down the sides of the bowl as needed.

Step 7

Add the eggs, one at a time, blending well after each addition. Add the vanilla and mix until incorporated. Scrape down the sides of the bowl and blend again.

Step 8

In a measuring cup with a spout combine the buttermilk and lemon juice. Add 1/3 of the flour mixture to the creamed butter and sugar. Beat on low until incorporated. Next, add 1/3 of the buttermilk mixture and blend until smooth. Add half the remaining flour, then half the buttermilk until both are incorporated. Finally, add the remaining flour, then buttermilk. Don't overmix. Using a spatula, turn the batter a few times scraping up from the bottom to ensure it's well blended.

Step 9

Pour the batter into the prepared pan and spread evenly. Sprinkle the crumble topping over the batter leaving some in large clumps. Bake in a preheated oven at 325°F for 40 to 50 minutes or until the top is lightly browned and a toothpick inserted in the center comes out with moist crumbs attached - but no wet batter. DO NOT OVER BAKE!

Step 10

Cool the cake on a wire rack for at least 15 minutes if serving warm, or cool completely before removing the sides of the pan. Carefully slide the cake onto a serving platter and dust with powdered sugar before serving. For a simple lemon glaze mix together ½ cup powdered sugar with enough fresh lemon juice for a drizzling consistency. Drizzle on top of the cake just before serving.