Adjust oven rack to middle position and heat oven to 200 degrees. When oven reaches 200 degrees F, shut off.
In the bowl of a stand mixer combine the milk and 1 tablespoon of granulated sugar; add yeast and let rest 10 minutes until foamy.
Add melted butter, eggs, flour, superfine sugar, vanilla, lemon zest and salt to the yeast mixture and mix with the dough hook on medium-low until thoroughly combined, about 10 minutes
Transfer the dough to a lightly oiled bowl, cover with plastic wrap and place in warm oven. Let rise until doubled in size, about 1 hour.
Cut the plums in half and quarter each half (getting a total of 8 slices per plum). Transfer to a large bowl and sprinkle with 2 tablespoons of granulated sugar and cinnamon. Stir and let rest for about 10 minutes.
After the dough has doubled in size turn it out onto a lightly floured surface, cover and let rest for 10 minutes.
Lightly grease and flour the bottom and sides of a 9-inch spring-form pan; line bottom with a 9-inch circle of parchment paper. Preheat oven to 350 degrees F.
Press dough into the prepared 9-inch spring-form pan. Arrange the plums in the pan, skin side down, to form a flower pattern; begin at the outside and work your way in to cover it completely. Let the cake rest for 20 minutes, then sprinkle with remaining 1 tablespoon granulated sugar.
Bake in a preheated oven at 350 degrees for 55 to 60 minutes, or until golden brown and toothpick inserted into the center of the cake comes out clean. Cool for 15 minutes, then unmold the cake by removing the sides of the pan. Slide a thin metal spatula under the cake to loosen, then slide the cake onto a wire rack to cool completely.
While the cake is still warm, heat the apricot preserves in a small saucepan over medium heat until completely melted. Remove from heat and add liqueur and stir until combined. Strain mixture through a fine strainer to remove any fruit lumps. Let cool until it is only slightly warm. Brush a light coat on the plums. Let cool completely before serving. Serve with a dollop of whipped cream.