Preheat the oven to 350 degrees.
Mix together the graham cracker crumbs, melted butter and sugar until it's the texture of wet sand. Pour into a 9-inch springform pan, then use the bottom of a cup to press it in to form a crust that comes 1 inch up the sides.
Bake for about 8 minutes, then remove from the oven and let cool.
Use an electric or stand mixer to beat the cream cheese and ricotta together until smooth. Add the sugar, eggs, 1 tablespoon lemon juice, vanilla, cornstarch, flour, melted butter, and sour cream. Mix until just combined and smooth, but try not to let it whip up too much or it will incorporate too much air.
Pour batter over the crust and smooth out the top. Carefully fan out the plums and arrange them in a pretty pattern on top. Sprinkle with remaining tablespoon of lemon juice.
Combine remaining tablespoon of sugar and cinnamon in a small bowl, then sprinkle over the cake.
Bake at 350 degrees for one hour. The center of the cake will still be quite jiggly. Turn off the oven, but leave the cake inside with the door closed for one hour more.
Let the cake cool down, then chill for at least several hours (overnight is best) before serving.