4ouncesrhubarb, trimmed, washed, and cut into 1-inch lengths
1/4cup(2 ounces) unsalted butter, at room temperature
1cup(4 ounces) confectioners' sugar
For the Custard Yo-Yos
1 1/3cups(6 ounces) all-purpose flour
1/2cup(2 ounces) custard powder (see baker's notes for sources and substitutions)
1/2cup(2 ounces) confectioners' sugar
3/4cup(6 ounces) cold unsalted butter, cut into 1-inch cubes
1/4teaspoonpure vanilla extract
For the Rhubarb Frosting
Center a rack in the oven to 350 (F). Line a small baking sheet with parchment paper and spread 4 ounces rhubarb out on the lined baking sheet. Roast for 30 minutes, or until softened. Remove from oven and allow to cool before transferring to the bowl of a food processor. Process to a puree, then add 1/4 cup butter. Sift in 1 cup confectioners' sugar, add 1/2 teaspoon lemon juice, and continue to process for a couple of minutes; it seems like a long time, but you want the frosting to thicken, which it will do as it's whipped. Transfer to a small bowl and chill in the fridge to firm up as you make the cookies.
For the Custard Yo-Yos
Center a rack in the oven and preheat to 325 (F). Line two baking sheets with parchment paper and set aside.
Combine 1 1/3 cups all-purpose flour, 1/2 cup custard powder, 1/2 cup confectioners' sugar, and 1/8 teaspoon kosher salt in the bowl of a freestanding electric mixer with a paddle attachment in place. Beat on low speed until combined. Add 3/4 cup cold, cubed unsalted butter and continue to beat on low speed until the mixture resembles breadcrumbs, about 3 minutes. Add 1/4 teaspoon vanilla extract and increase the mixer speed to medium, beating for about 30 seconds until the dough comes together.
Use a 1 tablespoon cookie dough scoop to portion out 24 cookie dough balls. Use your hands to roll them into perfect balls, like you did with Play-Doh when you were a tiny kiddo. Dip the prongs of a small fork in flour and gently but firmly press the prongs down into the middle of each cookie. The balls will increase to about 1 1/2 inches; don't press all the way down to the bottom, you just want to create firm lines in the dough rather than force them to spread out.
Bake for 25 minutes in the preheated oven; the cookies will be dry on the bottom but not 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 each cookie to a wire rack to cool completely.
Sandwich pairs of cookies together with the frosting, with the "forked" sides facing outward. You should use about 1/2 ounce of frosting in each cookie. It will seem like a lot, but the cookies can take it!