Preparation

1. Prepare a flat 18cm (7") disk of pâte sucrée. The ingredient amounts listed above will make two disks of this size; you can divide the dough in half and freeze the portion you’re not going to use.
2. Bake one 18cm (7") Sponge Cake Viennoise according to that recipe. Slice the cake into three layers and set aside.
3. If you’re using frozen blueberries, partially thaw them.
4. Begin preparing the gelatin for the blueberry gelatin and mousse in two separate small bowls or cups; put 1 and 2 tablespoons of water in each bowl, respectively, then sprinkle the gelatin into the water and set aside.
5. Prepare syrup; mix water and sugar and microwave until hot. Stir until completely dissolved, then let cool. Stir in fruit liqueur (such as Kirschwasser) and set aside.
6. Keep heavy cream chilled.

Directions

1. Make the blueberry mousse.
   
   1. Put the blueberries and sugar into a food processor and purée.
   2. Pour the heavy cream into a large bowl and whip until thick enough that cream dribbled from the beater briefly leaves a trail, but is not stiff.
   3. Microwave the prepared gelatin until it turns to liquid (but don’t boil it), then add to food processor along with the lemon juice and run the food processor again to mix.
   4. Pour the blueberry mix into the whipping cream and stir until mixed.
2. Assemble cake.
   
   1. Warm enough blueberry jam to coat one surface of the pâte sucrée disk in the microwave to soften it, then coat the top of the tart. Place a cake-forming ring onto the plate the cake will be served on (if the ring you’re using is adjustable, make it the same size as the sponge). Lay the base in the ring jam-side up.
   2. Lay the first layer of the sponge cake on top of the tart base, then dab on one third of the syrup.
   3. Pour one third of the mousse onto the sponge cake and spread smooth with a spatula.
   4. Lay the second layer of sponge cake onto the mousse and press down lightly. Dab half the remaining syrup onto the cake. Pour half the remaining mousse on top, again spreading smooth.
   5. Lay the top layer of sponge cake on top, press down lightly, and dab with remaining syrup. Pour on remaining mousse and spread evenly.
   6. Chill for about two hours in the refrigerator to set.
3. Decorate.
   
   1. Remove the cake ring from the cake.
   2. Whip one cup of heavy cream as described in the mousse directions above.
   3. Spread a thin layer of cream around the outside edge of the cake (leave the top bare), then put the cake and cream in the refrigerator to allow the layer to set. Add another coat of cream, chill, and repeat as many times as necessary to get a nice, even layer all the way around.
   4. Whip the remaining cream until stiff enough to hold its shape. Use a pastry bag to make a decorative wall of cream around the outside edge of the top; the gelatin will be poured into this, so make sure there aren’t any holes.
4. Make blueberry gelatin.
   
   1. Put the blueberries, white wine or water, and prepared gelatin in a saucepan and heat until the gelatin melts; don’t let it boil.
   2. Put the pan into cold water until the liquid just begins to thicken, then pour into the well on top of the cake, blueberries and all. Refrigerate the cake until the gelatin sets, about an hour.