• Preheat the oven to 350° F. Lightly spray a 8x8-inch square baking pan with nonstick spray, and then line it with parchment paper, making sure to leave a slightly overhang for easy removal. 

Lemon Crumble Topping

• In a medium bowl, mix together the all-purpose flour, granulated sugar, fresh lemon zest, and salt until combined.
  1/2 cup all-purpose flour, 1/3 cup granulated sugar, 1 teaspoon fresh lemon zest, ¼ teaspoon salt
• Add in the melted butter and mix with a fork until sandy crumbs form, with some larger clumps. Allow the crumbs to sit at room temperature while you prepare the cake. 
  3 tablespoons butter

Lemon Crumb Cake

• In a large mixing bowl or bowl of a stand mixer, using an electric mixer, beat the softened butter and granulated sugar at medium-high speed for 2 minutes, until creamy and just lightly fluffy. 
  1/2 cup (1 stick) butter, 3/4 cup granulated sugar
• Add the eggs, one at a time, and mix on low speed after each one just until combined. 
  2 large eggs
• Add in the lemon zest, fresh lemon juice, and vanilla extract and blend until smooth. 
  1 tablespoon fresh lemon zest, 2 tablespoons fresh lemon juice, 2 teaspoons vanilla extract
• In a separate bowl, whisk together the all-purpose flour, baking powder, salt, and baking soda until evenly combined. 
  1½ cups all-purpose flour, 1 teaspoon baking powder, ½ teaspoon salt, ¼ teaspoon baking soda
• Add the dry ingredients into the bowl of wet ingredients in two additions, alternating with half of the sour cream between each dry ingredient addition, mixing on low speed just until blended until no dry streaks remain. Avoid overmixing the batter to keep the cake tender. 
  1/2 cup full-fat sour cream
• Spread the lemon cake batter into the prepared baking dish and smooth out the top. Sprinkle the reserved crumble over the batter, making sure to distribute it evenly across the cake, to avoid the cake collapsing in one area. 
• Bake for 35-40 minutes, or until the center springs back lightly when touched and a cake tester or toothpick inserted into the center of the cake comes out clean with only a few crumbs (no wet batter!)
• Remove the cake from the oven and let it cool for 20 minutes at room temperature. Then lift the cake out using the parchment paper overhang and place it on a cooling rack. Allow the cake to cool completely. 

Lemon Glaze

• In a small bowl whisk together the powdered sugar, fresh lemon juice, and vanilla extract until combined. You want the glaze to be smooth, semi-pourable, and pipeable or spoonable. 

  ^{*Add a little more lemon juice if it is too thick, or a small amount of powdered sugar if it's too thin.}
  3/4 cup powdered sugar, 1-2 tablespoons fresh lemon juice, ½ teaspoon vanilla extract
• Pipe or drizzle the lemon glaze over the cake. Let the glaze set for a few minutes before slicing into 9 squares. Serve & enjoy!