Rhubarb Slab Cake with Crème Fraiche Frosting

2 3/4 cup all-purpose flour, plus more for dusting pans

2 1/2 teaspoons baking powder

1 teaspoon salt

1 cups unsalted butter

1 3/4 cups white granulated sugar

3/4 cup milk

6 large egg whites

2 teaspoons pure vanilla extract

1/2 cup roasted rhubarb (recipe follows)

 

Preheat oven at 350 degrees F and grease a 13×9-inch baking pan. Line with parchment paper and set aside. In a medium bowl, add the flour, baking powder and salt. Whisk until combined. Set aside. In a large bowl, combine sugar and butter and beat until light and fluffy with a hand mixer. In a small bowl, combine in the milk, egg whites, vanilla extract and 1/2 cup rhubarb puree and whisk until combined.  Add the dry ingredients into the butter and sugar mixture, in parts. Slowly add in milk/egg/rhubarb mixture and continue stirring until combined. Pour the batter into the prepared baking pan and transfer to the oven to bake for 25 minutes, until a knife inserted into the center comes out clean. Allow the cake to cool to room temperature. Top with Crème Fraiche Frosting and sprinkles!