Step 1

Combine 1/3 cup of the sugar and the zest of 1 lemon in a small food processor.

Step 2

Pulse until the zest is incorporated into the sugar and the sugar is a pale-yellow color.

Step 3

Melt 4 tablespoons of the butter in a small bowl.

Step 4

Add 3/4 cup of the flour, the lemon sugar, and 1 tablespoon of the lemon juice to the melted butter and mix with a fork to combine. Set the crumble topping aside.

Step 5

In a small food processor, add 1/2 of the remaining sugar and the remaining lemon zest and pulse until the sugar is a light-yellow color. Set aside.

Step 6

Preheat the oven to 325 degrees F.

Step 7

Invert the bottom of an 8-inch springform pan and lock on the sides of the pan. Turning the pan bottom upside down will make it easier to slide the cake from the pan.

Step 8

Using the extra butter and the extra flour, lightly grease and dust the bottom and sides of the pan.

Step 9

Line the bottom of the pan with a round of parchment paper.

Step 10

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

Step 11

In the bowl of a stand mixer fitted with the paddle attachment, combine the remaining butter, the remaining granulated sugar, and the lemon sugar.

Step 12

Beat the butter and sugar mixture until light and fluffy, scraping down the sides of the bowl as needed.

Step 13

Add the eggs, one at a time, to the butter mixture, blending well after each addition.

Step 14

Add the vanilla and mix until incorporated.

Step 15

Scrape down the sides of the bowl and blend again.

Step 16

In a measuring cup with a spout, combine the buttermilk and the remaining lemon juice.

Step 17

Add 1/3 of the flour mixture to the creamed butter and sugar mixture and beat on low until incorporated.

Step 18

Add 1/3 of the buttermilk mixture and blend until smooth.

Step 19

Repeat the adding and beating process until all of the flour and the buttermilk are incorporated, making sure not to overmix.

Step 20

Pour the batter into the prepared pan, spreading it out evenly.

Step 21

Sprinkle the crumble topping over the batter, leaving some of it in large clumps.

Step 22

Bake until the top is lightly browned and a toothpick inserted in the center comes out with moist crumbs attached, about 40-50 minutes.

Step 23

Cool the cake on a wire rack for at least 15 minutes.

Step 24

Carefully remove the sides of the pan and slide the cake onto a serving platter.

Step 25

Dust with the powdered sugar and serve.