Line a 40x30 cm baking tray with greased parchment paper. Sift all-purpose, cornstarch and baking powder. Beat the egg yolks with warm water until fluffy, then add in 100 grams of white sugar and vanilla sugar and beat until thick and light lemon colored. Beat the egg whites until foamy. Sprinkle in the remaining white sugar and beat until stiff peaks form. Fold yolk mixture into whites. Add the sifted dry ingredients to egg mixture, folding in carefully. Spread batter in prepared pan and bake at 200C/400F for 13 minutes or until just done. The center should spring back when lightly touched.
Turn out onto a clean dish towel (or a parchment paper) which has been liberally sprinkled with confectioners' sugar. Carefully peel off the paper and trim off crisp edges. While still hot roll cake with cloth from narrow end. Cool rolled up cake on a rack. When cool, unroll and spread jam over the cake. Roll up and chill for 1 hour. Cut into no thicker than ½-inch thick slices.
Make the berry mousse: In a blender puree berries with the honey and strain the puree through a fine sieve. Pour about 1/3 puree in a small bowl and sprinkle the gelatin powder over and let it soften for 5 minutes. Place mixture in a pot filled with some water. Heat until gelatin dissolves, stirring constantly, until the gelatin is dissolved and whisk it into the rest of the puree. Whisk the Philadelphia and yogurt into the strawberry mixture, whisking until it is smooth. Whip the cream until thick and fold into strawberry mixture.
Line a 8-inch plastic bowl with plastic wrap. Starting in the center, lay the cake slices edge to edge. The slices should be pressed together so any gaps between them are kept small. Continue this all the way around, up the sides of the bowl. Slowly pour the mousse in and cover with an 8-inch circle of cake. Chill overnight.
To serve: Place a cardboard cake-circle or a serving plate on top of the bowl and flip over. Tug gently on the plastic wrap to remove the cake from the mold. Garnish the cake with berries if desired.
