Gluten-Free Yellow Sheet Cake

Preparation

1. Step 1

   Make the cake: Place a rack in the center of the oven and heat to 325 degrees. Lightly butter the sides of a 9-by-13-inch baking pan (see Tips). Line the bottom and long sides with parchment paper.

2. Step 2

   Put the butter and sugar in a stand mixer bowl (or a large mixing bowl) and beat on medium-high speed with the paddle attachment (or a hand mixer) until light and fluffy, about 5 minutes. Add the eggs one at a time, letting each incorporate before adding the next. Add the vanilla and salt, and beat to combine well, scraping down the sides as needed.

3. Step 3

   Add the rice flour, millet flour and baking powder and beat on low until fully incorporated. Raise the speed to medium-low and slowly stream the milk in. The batter may start to look grainy or curdled; that’s OK. Scrape down the sides and bottom of the bowl and beat on medium speed for about 30 seconds.

4. Step 4

   Scrape the batter into the prepared pan and spread it into an even layer. Bake until a tester inserted in the center comes out clean, 37 to 40 minutes (start checking after 30 minutes if using a dark pan). Cool on a rack to room temperature.

5. Step 5

   If you’d like, make the frosting: In a small saucepan, whisk the rice flour and milk. Over medium heat, bring the mixture to a boil, whisking constantly, and continue to cook until it resembles a thick pudding, 1 to 2 minutes. Take the pan off the heat and whisk in the sugar, vanilla and salt. Transfer to a bowl and place reserved butter wrappers (see Tips) on the surface of the pudding to prevent a skin from forming; let cool to room temperature.

6. Step 6

   Once the pudding has cooled, beat the butter with a stand mixer and paddle attachment (or a hand mixer) until light and fluffy, 3 to 5 minutes. Little by little, add the pudding to the butter, making sure each addition is fully incorporated before adding the next. Once combined, scrape down the sides and bottom of the bowl, switch to the whisk attachment (if using a stand mixer), and whip for a few minutes, until the frosting is light and fluffy like thick whipped cream.

7. Step 7

   Immediately spread the frosting over the surface of the cake in an even layer using an offset spatula. Serve the frosted cake within a couple of hours or cover the pan with foil and refrigerate for up to 3 days.