For the Crumble Topping:
Melt 4 tablespoons of butter in a small bowl. Add the flour, sugar, lemon juice and zest. Mix with a fork to combine. Set aside.
For the Cake:
Preheat oven to 325°. 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.
In a small bowl combine the flour, salt and baking powder. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, combine the butter, lemon zest, and sugar. Beat for several minutes until light and fluffy. Scrape down the sides of the bowl as needed. Beat in the eggs and vanilla bean paste just until blended.
In a measuring cup with a spout, combine the buttermilk with the lemon juice. Add 1/3 of the flour mixture to the creamed butter and sugar. Beat on low until incorporated. Add 1/3 of the buttermilk mixture and blend until smooth. Repeat adding 1/3 of the flour, then 1/3 of the buttermilk until both are incorporated.
Pour the batter into the prepared pan and spread evenly. Sprinkle with the crumble topping (leaving some in large clumps) and bake in the preheated oven for 40 minutes or until the top is lightly browned and a toothpick inserted in the center comes out almost clean.
Cool the cake on a wire rack for at least 30 minutes if serving warm, or cool completely before removing the sides. Carefully slide the cake onto a serving platter and dust with powdered sugar before serving.
For a simple lemon glaze mix together 1/2 cup powdered sugar with enough fresh lemon juice for a drizzling consistency. Drizzle on top of the cake just before serving.