Martha makes vanilla sandwich cookies, a sweet way to end any holiday party.
In the bowl of an electric mixer fitted with the paddle attachment, mix together butter and sugar on medium speed until light and fluffy, about 3 minutes. Add eggs and salt; mix to combine. Add evaporated milk, vanilla extract, and vanilla bean seeds, if using; mix to combine.
Reduce speed to low; slowly add flour. Mix until just combined; do not overmix. Divide dough into 4 equal pieces and shape each piece into a disk. Cover tightly with plastic wrap. Transfer to refrigerator and chill until firm, at least 1 hour.
Preheat oven to 350 degrees. Line two baking sheets with nonstick baking mats or parchment paper.
Working with one disk at a time, form dough into 1-teaspoon-sized balls. Roll each ball in sanding sugar and place on prepared baking sheet, about 2 inches apart. Lightly dip the bottom of a drinking glass in flour and gently press down on ball to flatten slightly. Transfer baking sheets to oven and bake until puffy, but still soft, 8 to 10 minutes. Let cool slightly on baking sheets before transferring to a wire rack to cool completely.
Using a small offset spatula, carefully spread about 1 teaspoon filling on the flat side of one cookie; top with another cookie to make a sandwich. Repeat process with remaining filling and cookies.
Cookies can be kept in an airtight container at room temperature for 2 to 3 days.