Make the tart shells: In a medium bowl, combine the cream cheese, butter, and flour until it just comes together. (There should still be streaks of cream cheese.) Remove the dough from the bowl, pat into a disc, cover with plastic wrap, and refrigerate for at least 1 hour. Roll out the dough into 1/8-inch thickness and cut into 6 (3-inch circles). Refrigerate on a parchment-lined sheet pan for 15 minutes.
Make the filling: Dice 2 of the plums and toss together with 2 tablespoons of the granulated sugar, the cinnamon, and creme fraiche.
Assemble the tarts: Brush the dough lightly with the beaten egg and spoon a generous tablespoon of the filling in the center of each circle. Lift up the edges of the tart and pinch 3 points of the tart about halfway to the filling to form the sides of the crust. Refrigerate for 30 minutes. Brush the edges of the crust with egg and sprinkle lightly with the remaining 1 tablespoon granulated sugar. Pinch the dough again to make sure it is tightly sealed and freeze for 10 minutes. Remove the tarts from the freezer and immediately bake at 350 degrees for 20 to 25 minutes, or until golden brown. Let cool slightly and dust with confectioners' sugar.
Make the compote: Dice the remaining 2 plums and toss with simple syrup, lemon juice, and rosemary. Warm over medium heat for 2 to 3 minutes, until the plums are tender.
Make the Maple Mascarpone Cream: Whip the Mascarpone and heavy cream together until medium stiff peaks form. Add the maple syrup and whisk together until smooth.
Plate the tarts: Place 1 spoonful of plum compote in the center of each plate. Place a tart in the center of each plate and spoon some of the Maple Mascarpone Cream around the tart.