Preheat your oven to 325°F (163°C) and generously grease and flour an 8-inch springform or cake pan. Line the bottom of the pan with parchment paper and set it aside.

In a small bowl, prepare the crumble topping. Combine the melted butter, 90 grams (3/4 cup) flour, 67 grams (1/3 cup) sugar, lemon zest, and salt. Stir until well mixed, and then set it aside.

In a large mixing bowl, use an electric mixer to cream together the softened butter, 200 grams (1 cup) sugar, and lemon zest until light and fluffy.

Add the eggs, one at a time, followed by the vanilla extract, lemon juice, salt, and baking powder. Beat the mixture until all the ingredients are well combined.

With the mixer on low speed, gradually add half of the flour to the batter and mix until just combined. Then, add the sour cream and mix again until combined. Finally, add the remaining flour, followed by the milk, mixing until the batter is smooth and well incorporated.

Pour the batter into the prepared pan, spreading it evenly. Sprinkle the crumble topping over the batter, making sure to leave some larger chunks for added texture.

Bake the cake in the preheated oven for approximately 40 minutes or until the top turns a light golden brown and a toothpick inserted into the center comes out clean.

Once baked, remove the cake from the oven and allow it to cool on a wire rack for at least 30 minutes before serving. If desired, dust the top with powdered sugar for an elegant finishing touch. Slice and enjoy!