We’re not lying when we say this dessert is magical. The cake recipe is absurdly moist and tender. And the pastel frosting? It’s basically the silky smooth love child of Swiss meringue buttercream and whipped cream. Also, the icing is substantially less sweet than the traditional butter and icing sugar mixture, so you can eat more than one slice. Plus those rainbow sprinkles! So, yeah, it’s straight-up magical.
This recipe requires 1 hour cool time.
Preheat oven to 325°F. Spray a 9 x 13-inch baking pan with cooking spray and line with parchment paper with slight overhang on the long sides.
In bowl, whisk together flour, baking powder and salt.
In bowl of stand mixer fitted with a whisk attachment, beat eggs, egg whites, sugar and vanilla on medium-high speed until thick and batter falls into a ribbon pattern, 3 to 4 minutes. Stir in flour mixture until just combined.
Meanwhile, heat milk in small saucepan over medium-low heat until just hot. Remove from heat; stir in butter and oil until melted. Stir into flour mixture until just combined. Stir in sprinkles.
Scrape batter into prepared pan, spreading evenly. Bake until edges are lightly brown and a toothpick inserted into centre of the cake comes out clean, about 45 minutes. Let cool in pan 15 minutes. Transfer to rack right side up and let cool completely, 45 minutes.
In medium saucepan, whisk together flour and milk for the frosting, until smooth. Place over medium heat and whisk constantly until thickened, about 2 minutes. Remove from heat; whisk in sugar and salt. Scrape into a wide shallow plate to cool (stir until slightly warm).
Beat butter in bowl of standing mixer fitted with paddle attachment on medium-low for about 5 minutes. Scrape the bottom; beat 1 minute. Gradually add cooled flour mixture, 2 spoonfuls at a time on medium-low until combined. Scrape bottom; beat until smooth.
Switch to whisk attachment and whip on low just to combine. Increase speed to medium-high and whip until silky and light. Frosting should have an airy consistency. Add desired food colour, stirring until incorporated. Spread on cake immediately with offset spatula to create swoops and pipe shapes (if desired) using a piping bag fitted with a star or crown piping tip. Add rainbow sprinkles.
Tip: Don’t be alarmed if the frosting feels stiff or looks curdled, your flour mixture was likely too cold. To warm, set the mixing bowl over a pot of simmering water until the edges start to melt; then rewhip. If the frosting looks loose, it’s likely too warm. Refrigerate for 10 minutes, stir and then rewhip until silky.