1. Preheat oven to 350°. Coat a large baking sheet with cooking spray.
2. Combine flour, 1/2 cup sugar, baking powder, and salt in a food processor. Add ricotta and next 4 ingredients (through vanilla); pulse until mixture begins to form a ball. Do not overprocess. Shape into a disk, wrap in plastic wrap, and refrigerate 2 hours or overnight.
3. Place dough on lightly floured wax paper. Top with another sheet; roll out to a 10-inch circle. Remove wax paper, and transfer dough to prepared baking sheet. Fold edges to make a 1/2-inch rim.
4. Bake for 10 minutes in center of oven. Remove from oven, and brush top with egg white. Starting at outer edge and working toward the center, arrange plum slices in smaller and smaller circles until crust is covered. Sprinkle 2 tablespoons sugar evenly over plums, and bake for 25 minutes. Top with almonds and remaining 1 tablespoon sugar, and bake for 10 minutes longer or until crust is golden brown and plums are softened. Remove tart from the oven, and cool for at least 5 minutes. Cut into wedges and serve.