1) Preheat oven to 325° Grease a 9-inch round cake pan. In a small bowl, combine the sugar & melted butter. Spread in the bottom of the prepared pan. Layer with the rhubarb & sprinkle with 4 1/2 teaspoons of maple sugar. Set aside.
2) In the bowl of a stand mixer fitted with the paddle attachment, cream the butter & 3/4 cups of maple sugar until light & fluffy, about 5 minutes. Add the egg yolks & vanilla extract and mix.
3) In a small bowl, whisk together the flour, baking powder, and salt. With the mixer running on low, slowly add half of the flour, the milk, and the other half of the flour, beating well after each addition & scraping down the sides of the bowl as necessary.
4) In a clean, medium-sized bowl, beat the egg whites with the cream of tartar until stiff peaks form. Gradually fold into the batter. Gently spoon over the rhubarb in the pan.
5) Bake in preheated oven until the 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.