Preheat oven to 425°F. In a large bowl, combine flour, sugar and salt. Sift in the baking powder to assure there are no lumps.
In a separate bowl, cream the butter and cream cheese with a mixer until light and fluffy. Scrape down the sides and add the flour mixture and mix just until the dough comes together.
Transfer the dough to a lightly floured board and knead until smooth, about 2-minutes.
Wrap in plastic wrap and chill in the refrigerator until slightly firm, but not hard. About 30 minutes.
With a rolling pin, roll out ball of dough on lightly floured surface into a disc slightly larger than the circumference of the tart pan. Transfer dough to tart pan, press dough gently into pan and trim excess edges. Bake tart shell for 10 minutes.
While tart shells are baking, make filling:
Drain Plums and reserve the juice.
Slice Plums in half and pit.
Whisk together sugar and cornstarch in a large bowl. Add 3 tablespoons plum juice and lemon juice and mix thoroughly.
Remove tart shell from oven. Lower temperature to 375.
Arrange plum halves, cut sides down, in circular pattern in tart shells. Pour plum sauce over plums.
Bake tart for 30 minutes. Cook until juices are bubbling and slightly thickened. If the edges of the crust start to darken too quickly, cover them with aluminum foil.
Cool tarts completely in pans on a rack before unmolding.