All Ups, No Downs: Custard Yo Yos with Roasted Rhubarb Icing

A few weeks ago I remembered that Yotam Ottolenghi and Helen Goh's recipe for "Custard Yo-Yos with Roasted Rhubarb Icing" was on my to-bake list -- it's the very first recipe in their cookbook Sweet -- and I managed to buy a few rhubarb stalks at the very end of the season. The "custard" descriptor refers to the custard powder in the cookie dough.

I made the rhubarb icing first by cutting up the rhubarb; roasting it in the oven until soft; and blitzing it in the food processor along with softened butter, powdered sugar, and lemon juice. I also threw in a dash of salt. While the rhubarb icing in the cookbook photo is a striking magenta color, mine was a washed out pinkish-beige hue. For a second I thought about adding some dye, but I decided to just go with it. Also, the recipe says that the filling requires several hours of chilling to firm up to the proper texture. My icing was not runny at all and it seemed like it was already the perfect texture to use as a sandwich cookie filling. But I covered it in plastic wrap and stuck it in the fridge while I made the cookies.

Making the cookie dough is a breeze. You put all of the dry ingredients (flour, custard powder, powdered sugar, and salt) in a mixing bowl; add softened butter and beat until the mixture resembles crumbs; and add vanilla extract and mix until the dough comes together. The recipe says that it will produce 30 individual cookies that are about 15 grams each, but I wanted to make cookies that were a little larger. Using a #50 scoop, I portioned out cookies that weighed roughly 19 grams, and I got 45 cookies from a double batch of dough. I rolled each ball of dough between my palms to make it perfectly smooth before placing it on a parchment-lined baking sheet and marking the top with the tines of a large fork. I was delighted that all of the cookies baked into identical perfectly round domes. The were so uniform that they actually didn't look like they were homemade.
The cookbook photo is an overhead shot of cookies in various stages of assembly, including some cookies topped with rhubarb icing but still waiting for another cookie to be placed on top. On these open-face cookie sandwiches, the icing looks like it has been clumsily applied with an offset spatula. The haphazard look of the icing in the photo bothered me, so I decided to put my rhubarb icing in a pastry bag fitted with a large plain tip and pipe it on. The icing was quite stiff after having been in the fridge for a few hours, so I had to massage the filled pastry bag to loosen up the icing and make it easier to pipe. I'm glad I took the extra step of piping the filling, because the assembled cookies looked nice and neat.

I did not fully appreciate -- until I first bit into a filled cookie sandwich -- that the cookies were actually domed quite high. In retrospect, I probably should have stuck with the smaller cookies specified by the recipe, or perhaps flattened the cookies more before baking, because my sandwiches contained an awful lot of cookie. That said, they were quite tasty. I loved the filling, which was sweet and fruity. I could not specifically identify the flavor as rhurbarb (and neither could my tasters, several of whom asked me what was in the filling), but if I had to describe it, I would say it tasted like some sort of berry. I was expecting the cookies to be tender, since custard powder is mostly cornstarch. But they were quite firm, and biting into a cookie sandwich created a lot of fine crumbs. The upside is that the filled cookies didn't soften, even after several days.

I would make these cookies again, and the recipe is a good way to use up leftover rhubarb; the recipe requires only 70 grams of rhubarb, about one stalk. Since you roast the rhubarb and blend it with a lot of sugar, the recipe is also an excellent way to use rhubarb that might not be at its peak -- these cookies are probably the best thing I could have made with the end-of-season rhubarb.

Recipe: "Custard Yo-Yos with Roasted Rhubarb Frosting" from Sweet by Yotam Ottolenghi and Helen Goh.