In a blender or food processor, purée the cranberries until smooth.
In a pot off the heat, combine the sugar and cornstarch. Add the eggs and egg yolks. Whisk to combine. Add the puréed cranberries and the lemon juice. Cook over medium heat, stirring constantly, scraping the bottom and sides of the pot until the pastry cream thickens.
Transfer to a bowl. Cover with plastic wrap directly on the surface of the cream. Let cool. Refrigerate for 3 hours or until completely chilled. When ready to use the pastry cream, stir with a spatula to loosen.
With the rack in the middle position, preheat the oven to 325°F (165°C). Line the bottoms of two 8-inch (20 cm) springform pans with removable bottoms with parchment paper. Do not butter the sides.
In a bowl, combine the flour and baking powder.
In another bowl, whisk the eggs, sugar and vanilla with an electric mixer until the mixture has tripled in volume and falls from the beaters in a ribbon, about 10 minutes. With the machine running on low speed, gradually add the oil. Using a whisk, delicately fold in the flour mixture.
Using a whisk, fold in the chopped cranberries. Divide the batter between the cake pans.
Bake for 40 minutes or until a toothpick inserted in the centre of a cake comes out clean. Remove from the oven and immediately turn over onto a wire rack to cool completely, about 3 hours. Pass a thin blade between the sides of the pans and the cakes and unmould.
Sour Cream Icing
In a bowl, whisk the butter and icing sugar with an electric mixer until creamy, about 5 minutes. Using a spatula, fold in the sour cream.
Cut each cake in half horizontally to have a total of 4 cake layers.
Place one cake layer on a serving dish. Top with one-third of the pastry cream. Repeat with two of the remaining cake layers and the remaining pastry cream. Top with the final cake layer. Using an offset spatula, cover the entire cake with the sour cream icing.
Decorate the cake by planting the rosemary sprigs upside down in the top of the cake to resemble little pine trees. Sprinkle with icing sugar. Add sparkling sugar crystals to the outside of the cake.
Refrigerate for 1 hour before serving. The cake will keep for 2 to 3 days, covered, in the refrigerator.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.