Custard Creams

Instructions

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1. 1 Line a 13- by 18-inch rimmed baking sheet with parchment; set aside.
2. 2 For the Biscuits: In a medium bowl, whisk together all-purpose flour, custard powder, and salt. Set aside.
3. 3 In the bowl of a stand mixer fitted with the paddle attachment, beat butter and sugar on medium-high speed until creamy, about 1 minute. Add egg yolk and vanilla bean paste or extract and beat on medium-high speed until fully incorporated, about 20 seconds. Add dry ingredients and mix on medium speed until dough just comes together, about 45 seconds, pausing halfway through to scrape bowl with a flexible spatula. With the mixer running, add milk and mix until a thick dough forms, about 10 seconds. (Alternatively, you can prepare the dough using an electric hand mixer.)
4. 4 Use your hands to gently press the dough together; it will feel soft and slightly sticky. Flatten into an 8-inch disc, wrap tightly with plastic wrap, and refrigerate until firm, about 1 hour.
5. 5 On a generously floured surface, roll chilled dough to about 5-mm thickness. With a 1 1/2- by 2-inch rectangular cutter, cut biscuits and use an offset spatula to transfer to prepared baking sheet, leaving at least 3/4-inch between each. Gather scraps, knead, re-roll, and cut as before. (If dough feels warm or sticks to the rolling pin, refrigerate for 10 minutes or until firm enough to roll.) You should have 30 rectangles. Refrigerate baking sheet with cookies for 30 minutes. (If a 13- by 18-inch rimmed baking sheet won’t fit in your refrigerator, you can bake the biscuits on two or three 9- by 13-inch rimmed baking sheets instead.)
6. 6 While dough chills, adjust oven rack to middle position and preheat oven to 350°F (175°C). Bake biscuits until golden with very lightly brown edges, about 13 minutes, then cool on baking sheet for 5 minutes. Using an offset spatula, transfer biscuits to a wire rack to cool completely while you prepare the buttercream.
7. 7 For the Buttercream: In a small microwave-safe bowl, heat milk on medium-high power until very warm to the touch, 8 to 10 seconds. Add custard powder and whisk until a thin yellow paste forms, 5 to 10 seconds.
8. 8 In the bowl of a stand mixer fitted with paddle attachment, whip butter on medium-high speed until smooth and creamy, about 30 seconds. Add confectioners’ sugar, milk and custard powder mixture, and vanilla paste. Mix on medium-high until smooth and creamy, about 1 minute. The buttercream should be soft and creamy, but firm enough to hold its shape when piped. If it feels too thin, add powdered sugar, 1 teaspoon at a time, until thick enough to pipe. Using a flexible spatula, scrape buttercream into a piping bag.
9. 9 Using scissors, cut a 1/2-inch hole in the piping bag. Pipe 1 to 2 teaspoons of buttercream onto the flat side of half the biscuits, then sandwich with remaining biscuits.