Preheat your oven to 325F
Sift flour, custard powder (or cornstarch), powdered sugar, and salt into a mixer bowl with paddle attachment in place. Combine on low speed briefly. Add unsalted butter and continue to beat on low speed until mixture resembles coarse cornmeal.
Add vanilla extract and increase the mixer speed to medium, beating for about 30 seconds until dough comes together.
Forming the cookies – your dough may seem too dry, not to worry, it will come together. Using your hands, grab about a tbsp worth of dough, then squeeze, and roll the dough together in your hands as you would play-dough until it resembles a ball. It will be approximately 1 inch in diameter. I use a #60 ice cream scoop to dole out the dough. You should have about 30 balls.
Place on a parchment-lined baking sheet about 1.5 inches apart, using a fork lightly dipped in flour, gently but firmly press the prongs down into the middle of each cookie (not all the way through). Cookies will be about 1.5 inches in diameter with pronounced grooves from the fork.
Bake for 25 minutes; the cookies will be dry on the bottom but should not be too browned. They will be relatively fragile when warm but still firm to the touch. Leave the cookies on their baking sheets to cool for 5 minutes before transferring to a cooling rack.
Sandwich pairs of cookies together with the buttercream, with the “forked” sides facing outward. Use approximately 1/2 ounce (15gm) of icing per cookie.