Rhubarb Upside Down Cake

1. Preheat oven to 325°F. Grease a 9-inch round cake pan. In a small bowl, combine 2/3 cup sugar and melted butter. Spread in the bottom of prepared pan. Layer with rhubarb and then sprinkle with 4 1/2 teaspoons of maple sugar. Set aside.
2. For the batter, in the bowl of a stand mixer fitted with paddle attachment, cream butter and maple sugar until light and fluffy, about 5 minutes. Add egg yolks and vanilla extract and mix.
3. In a small bowl, whisk together flour, baking powder, and salt. With mixer running on low, into creamed mixture, slowly add half of the flour, then milk, and then the other half of the flour, beating well after each addition and scraping down sides of the bowl as necessary.
4. In a clean, medium-sized bowl, beat egg whites with cream of tartar until stiff peaks form. Gradually fold into batter. Gently spoon batter mixture over rhubarb in pan.
5. Bake in preheated oven until cake springs back when lightly touched, about 50–60 minutes. Cool in pan on a rack for 10 minutes before inverting onto a serving plate. Serve warm with whipped cream or vanilla custard.

Recipe adapted from Taste of Home.