Make the dough: In a large bowl, beat butter and sugar until light and fluffy, about 2–3 minutes. Add the egg and vanilla, and mix until combined.

Combine dry ingredients: Whisk flour, cornstarch, baking powder, and salt. Add to the wet mixture on low speed until a soft dough forms. Don’t overmix.

Chill: Divide dough in half. Flatten into discs, wrap, and chill for 45–60 minutes, until firm but rollable.

Preheat and prep: Heat oven to 350°F (175°C). Line 2 baking sheets with parchment.

Roll and cut: On a lightly floured surface, roll dough to about 1/8–1/4 inch (3–6 mm) thickness. Cut into 1 1/2–2 inch rounds (or shapes). Re-roll scraps as needed.

Bake: Arrange cookies 1 inch apart. Bake 8–10 minutes until edges are set and just turning pale gold. They should not brown deeply. Cool on the sheet 5 minutes, then transfer to a rack to cool completely.

Make the filling: Beat butter until creamy. Add powdered sugar, vanilla, and salt; mix on low, then beat in cream 1 tablespoon at a time until smooth and spreadable.

Assemble: Pair cookies by size. Pipe or spread 1–2 teaspoons of filling on the underside of one cookie. Top with its partner and press gently to nudge the filling to the edges.

Set: Let the sandwiches rest 15–20 minutes so the filling firms up before serving.