Combine the flour, sugar, and lemon zest. Add the butter and mix with a fork or your fingers until the mixture forms crumbs.

1. Refrigerate the topping while you make the rest of the cake.

TO MAKE THE FILLING:

1. Preheat the oven to 350°F. Grease an 8-inch square baking pan.
2. Using an electric mixer on medium speed, beat the cream cheese and sugar until thoroughly combined. Mix in the egg and lemon juice. Set aside.

TO MAKE THE CAKE:

1. Whisk together the flour, baking powder, salt, and lemon zest. Set aside.
2. Using an electric mixer on medium speed, beat the butter and sugar until light and fluffy. Add the egg and lemon juice, and mix well.
3. Reduce mixer speed to low. Add the flour mixture in three portions, alternating with two portions of milk. Mix just until combined.
4. Transfer about half of the cake batter to the prepared pan, and spread evenly.
5. Top with the filling mixture, and carefully spread it to cover almost to the edges.
6. Dollop the remaining cake batter on top of the filling. Gently spread the batter to cover the filling.
7. Sprinkle the topping mixture evenly over the top of the cake.
8. Bake 30 to 35 minutes, or until a pick inserted into the center comes out clean. Cool in the pan on a wire rack before serving.