1. Preheat the oven to 350°F and line two baking sheets with parchment paper.

2. Whisk together the flour, baking powder, and salt in a medium bowl.

3. In a large bowl, beat the butter and granulated sugar until light and fluffy.

4. Add the egg, lemon zest, lemon juice, and vanilla extract, then mix until smooth.

5. Add the dry ingredients and mix just until a soft dough forms.

6. Chill the dough for 30 minutes to make it easier to shape.

7. Scoop tablespoon-sized portions, roll into balls, place on the baking sheets, and flatten slightly.

8. Bake for 9 to 11 minutes, until the cookies are set and the edges are lightly golden.

9. Cool on the pan for 5 minutes, then transfer to a wire rack to cool completely.

10. For the filling, beat the cream cheese and butter until smooth.

11. Mix in the powdered sugar, lemon zest, lemon juice, and enough cream or milk to make the filling fluffy and spreadable.

12. Spread or pipe the filling onto the flat side of half the cookies, then top with the remaining cookies to make sandwiches.

13. Chill for 15 to 20 minutes before serving if you want the filling to firm up slightly.