Double-Berry Sammies

In a small bowl, whisk together the flour and baking powder; set aside. In a large bowl, combine 1 cup butter, the granulated sugar and 1 teaspoon kosher salt. Using the tip of a small sharp knife, split vanilla beans lengthwise and scrape seeds into the bowl. Beat with an electric mixer on medium speed until light and fluffy. Add the eggs and 1 teaspoon vanilla, beating until just combined. Add the flour mixture, beating on low speed just until combined.

Form dough into 60 balls (about 2 teaspoons each) and place on a baking sheet lined with parchment paper. (If desired, roll balls in colored sugar first.) Freeze for 20 minutes. Meanwhile, preheat oven to 375°.

Arrange chilled balls 2 inches apart on baking sheets lined with parchment paper and bake for 10 to 12 minutes, until edges are just set. (Centers should still be soft.) Transfer to a rack; cool completely.

For buttercream: In a large bowl, beat the remaining 1 cup butter, the powdered sugar, the remaining 1/4 teaspoon kosher salt and the remaining 1 teaspoon vanilla on medium speed until light and fluffy. Add strawberry jam and beat on high for 2 minutes. Add enough milk, 1 teaspoon at a time, for the buttercream to reach a soft, creamy consistency.

To assemble: Leaving a small border, spread about a teaspoon of blueberry jam on the undersides of half the cookies and about 2 tablespoons of buttercream on the undersides of the rest. Gently sandwich one of each cookie together and chill until firm, about 20 minutes or up to 4 days.