Meyer Lemon Whoopie Pies

Preheat oven to 350°F and line 2 baking sheets with silicone mats or parchment paper. Whisk together flour, baking soda, and salt in a medium bowl. Beat butter and sugars in a large mixer bowl on medium speed until pale and fluffy. Beat in lemon zest and egg until combined. Reduce speed to low and add half of flour mixture to sugar mixture; beat just until combined. Add milk and vanilla, then finish with remaining flour mixture.

Drop 56 mounds of dough (about 2 teaspoons each) 2 inches apart onto prepared baking sheets. Bake until tops are golden and spring back when lightly touched, about 9 to 10 minutes. Cool for 2 minutes on baking sheets, then transfer cookies to a wire rack to cool completely. If making cookies a day ahead of filling, store at room temperature in an airtight container between layers of parchment paper.

For the filling, beat mascarpone and lemon zest with a mixer on medium speed until smooth. Reduce speed to low and gradually add powdered sugar until combined; mix in milk just until filling is fluffy. Place half the cookies on a baking sheet and spoon a heaping teaspoon of filling on the flat side of each, then sandwich with remaining cookies, pressing gently.

Store filled whoopie pies in the refrigerator up to 1 day.