1. 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.

2. 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.

3. Preheat oven to 350 degrees. Line two baking sheets with nonstick baking mats or parchment paper.

4. 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.

5. 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.