In a small food processor, blend the granulated sugar with the lemon zest until the sugar adopts a pale yellow hue.

In a separate bowl, melt the unsalted butter. Following that, incorporate the previously processed lemon sugar, all-purpose flour, and lemon juice. Mix thoroughly and set aside.

Utilize the same food processor and combine half the granulated sugar with the zest from the two lemons. Process until fully integrated.

Preheat your oven to a temperature of 325°F (163°C). Prepare an 8" springform pan by inverting its bottom, securing its sides, and then lightly greasing and flouring its surfaces. Place a round parchment paper on the bottom.

In a distinct mixing bowl, sift and combine the all-purpose flour, salt, and baking powder. Set aside.

Utilizing a stand mixer equipped with a paddle attachment, integrate the unsalted butter, the remaining granulated sugar, and the processed lemon sugar. Continue mixing until the texture is light and airy.

Introduce the eggs into the mixture one at a time, ensuring each is fully incorporated before adding the next. Follow with the addition of vanilla and mix thoroughly.

In a separate measuring vessel, blend together the buttermilk and lemon juice. Begin adding ⅓ of the flour mixture to the primary mixture, blending on low speed. Following this, add ⅓ of the buttermilk mixture and continue to blend. Repeat this process until all the buttermilk and flour have been integrated. Avoid overmixing.

Transfer the completed batter to the prepared pan, ensuring it is spread uniformly. Carefully apply the crumble topping over the batter, ensuring larger clumps are evenly distributed.

Bake the cake in the preheated oven for approximately 40 to 50 minutes or until the cake achieves a light brown hue and a toothpick inserted into its center emerges with moist crumbs, but without any raw batter. Ensure the cake is not overcooked.

Allow the cake to cool on a wire rack for a minimum of 15 minutes if intending to serve warm, or let it cool entirely before extracting it from the pan. Once ready, transfer the cake onto a serving dish, and if desired, dust with powdered sugar prior to serving. For additional flavor, one may prepare a simple lemon glaze by combining ½ cup of powdered sugar with an adequate amount of fresh lemon juice to achieve a drizzling consistency. Apply this glaze atop the cake just before serving.