Preheat the oven to 400°F. Place sweet potato on a foil lined pan. Bake for 45 minutes or until tender when tested with a fork. Let it cool to room temperature. Remove the peel and mash the sweet potato. You should have 3/4 cup potato puree.Coat an 8-inch springform pan with baking spray or line an 8-inch cake pan with aluminum foil or parchment paper, if desired, and coat with baking spray.
Reduce oven to 325 degrees.
Separate the eggs; place the egg yolks in a mixing bowl with the sugar and beat on high speed with an electric mixer for 2 minutes, or until lemon-colored. On low speed, gradually add the melted butter, mashed sweet potato, and vanilla, scraping down the sides as needed. Add the flour and mix just until flour is combined. Gradually add the milk, beating just until combined.
If using the same mixer and beaters to beat the egg whites, wash the beaters thoroughly; place the egg whites in a separate mixing bowl. Beat on medium speed until foamy; add the cream of tartar. Increase the speed to high and beat until the egg whites form stiff peaks, about 2 minutes. Gently whisk about one-third of the egg whites into the thin batter; gently fold the remaining egg whites in with a silicone spatula. Pour the batter in the prepared cake pan. Bake at 325 degrees for 38 minutes, or until the center jiggles slightly when gently shaken.
Let cool completely on a wire rack, about 1 hour. Cover and refrigerate at least 2 hours to chill. Remove sides and bottom of springform pan and transfer cake to a serving plate.
Beat whipping cream and 1 tablespoon sugar in a mixing bowl at medium-high speed until stiff peaks form; spread it on top of cake. Garnish with edible flowers such as Oxalis or shamrock blooms.