Preheat the oven to 350F/177C. Line a large, 13x18-inch (33x46-cm) baking sheet with parchment paper and leave the sides ungreased.
Prepare the sponge cake first. Place the eggs, vanilla and sugar into a mixer bowl and whisk on high speed for 8 to 10 minutes, until the eggs are thick, fluffy and pale creamy in color. If using a hand mixer, increase mixing time by 3 to 5 minutes. In a separate bowl, combine the flour and baking powder. Sift the flour into the egg mixture in small batches, folding it in gently but thoroughly after each addition. Fold from the bottom of the mixing bowl to ensure all the flour gets incorporated.
Spread the cake batter evenly to the edges of the prepared baking sheet using a spatula. This cake will not self-level so spreading it evenly is important. Bake the sponge cake in the preheated oven for 16 to 18 minutes, until it’s a light, golden color on the top.
Immediately after removing the cake from the oven, run a knife along the edges of the pan to release it and sprinkle the top with confectioner’s sugar. Invert the cake onto a wire rack (watch my video to see how it’s done), then remove the parchment paper. Place a clean kitchen towel or dinner napkin on top of the cake, then gently and loosely roll the cake, lengthwise. Allow the cake to cool completely on the wire rack before assembling.
For the mascarpone whipped cream, place the softened cheese, confectioner’s sugar and vanilla into a large mixing bowl. Whisk for a couple minutes, until the cheese is smooth. Add ½ cup chilled heavy cream and mix for about 30 seconds, to thin out the cheese mixture. Next, add the remaining heavy cream and whisk for 4 to 5 minutes on high speed, until stiff peaks form. Keep the frosting refrigerated if not using right away.
To assemble the cake, first gently unroll the cake and remove the dinner napkin. Spread the crushed strawberries onto the sponge cake, evenly to all the edges. Next, add two-thirds of the prepared mascarpone frosting and spread it evenly to the edges using an off-set spatula. Do not over fill the cake, as this can cause it to break. Sprinkle the diced strawberries evenly over the mascarpone cream.
Gently roll the cake back up the same way it was roll initially, supporting the back of the cake roll with your hand. Watch my YouTube video to see how I do it.
Frost the outside of the cake with the remaining frosting and garnish with dollops of cream. I used French star tip #4 for my cake. Garnish with more fresh strawberries on top.
Refrigerate the cake for at least 4 to 6 hours, or overnight for best results. This will allow the frosting to set nicely and the sponge cake will become more moist from the cream and crushed berries.