Make the Spiced Honey: Mix together the honey, cinnamon, cardamom, and orange zest in a medium saucepan. Warm the honey over medium low heat until the honey is hot and liquid. Remove the honey from the heat and let it cool. The honey can be made the day before - once it's cool, transfer it to a sealed container and set it aside.
Make the Plum Tarts: Preheat the oven to 375F.
Grease a standard 12 well muffin tin. Unfold one of the pastry sheets. Dust it liberally with flour and give it a quick roll with a rolling pin to flatten it out. Then divide the sheet into 3 strips. Roll each strip until it is about 4 1/2 inches by 12 inches. Cut the strip in half to make two 6 inch long pieces. Repeat for the remaining strips to make 6 pieces in total. Arrange each piece into a well of the muffin tin so that it lines the well. Take out the second pastry sheet and do the same for it.
Slice up the plums and divide them among the 12 pastry pieces in the muffin tin. Dab the corners of a pastry piece with water and then pinch the four corners together and also pinch the seams to make a nice package. Repeat for each until you have 12 sealed tarts.
In a small bowl, mix together the egg and milk for the egg wash. And, in another small bowl, stir together the sugar, cinnamon, and cardamom. Brush the tops of each tart with egg wash and then sprinkle with 1 teaspoon of the sugar topping.
Put the assembled tarts in the freezer for 10 minutes. Then bake the tarts for 25 minutes in the oven. When they are done, let them sit for about 1 minute, then carefully scoop each tart out of the tin onto a cooling rack. Don't wait too long - any juices that escaped the tarts will stick the tarts to the tin if they are left in the tin.
Make the Mascarpone Cream: In a medium bowl, whip the mascarpone with 1/4 cup of the spiced honey. Set that aside. Pour the cream into a large bowl and beat with a mixer on high until you have stiff peaks. Add the mascarpone to the cream and beat it together.
To serve, drizzle the tarts with some of the spiced honey and then scoop a dollop of mascarpone alongside. Tarts can be served warm or room temperature.