In the same small food processor used above, again, add half the granulated sugar and the zest from two lemons and pulse/process until combined and the sugar is a light yellow color.
Preheat oven to 325°F. Invert the bottom of an 8" or 9" 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.
In a small bowl combine the flour, salt and baking powder. Set aside.
Using mixer, combine the butter, remaining granulated sugar and the lemon sugar. Beat for several minutes until light and fluffy.
Add the eggs, one at a time, blending well after each addition. Add the vanilla and mix until incorporated.
In a measuring cup with a spout combine the buttermilk and lemon juice. Add ⅓ of the flour mixture to the creamed butter and sugar. Beat on low until incorporated. Next, add ⅓ 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.
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 overbake.
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.