Preheat your oven to 350°F. Grease and flour a 9-inch springform pan or square baking dish.
In a small bowl, whisk the flour, sugar, salt, and lemon zest. Pour in the melted butter and stir with a fork until the mixture forms thick, moist clumps. Set aside to let the flour absorb the butter while you make the batter.
In a medium bowl, beat the softened cream cheese, sugar, egg yolk, and lemon juice until completely smooth.
In a large bowl or stand mixer, cream the softened butter and sugar until light and aerated (about 3–4 minutes). Add the eggs one at a time, then mix in the lemon juice, zest, vanilla, and sour cream until smooth.
Whisk the flour, baking powder, baking soda, and salt. Gently fold the dry ingredients into the wet mixture until just combined. Be careful not to overmix, as this can make the cake tough.
Spread about two-thirds of the batter into the prepared pan. Spoon the cream cheese filling over the center, leaving a small border around the edge. Cover with the remaining batter.
Use your hands to break the streusel into chunky pieces and scatter them over the top. Bake for 45–55 minutes. A tester inserted into the cake portion should come out clean.
Allow the cake to cool in the pan for at least 20 minutes before removing or slicing.
Whisk together until smooth and drizzle over the cooled cake.
