Prepare jelly roll pan by lining it with waxed paper or parchment paper, and butter the paper. I just use a cookie sheet with edges, it doesn't have to be a jelly roll pan.
Preheat the oven to 375, and separate the 4 eggs.
Whip the 4 egg whites until stiff, then pour onto a plate and set aside. I always do the egg whites first, because then I don't have to wash out the mixer bowl.
4 eggs
Put the 4 egg yolks in a mixer bowl with the sugar and beat until creamy. Add the vanilla and beat again.
3/4 cup sugar, 1 teaspoon vanilla
In a separate bowl combine the flour, salt and baking powder.
3/4 cup flour, 1/4 teaspoon salt, 1 teaspoon baking powder
Now fold the egg whites into the egg yolk mixture while sifting the flour over the top, stirring gently with a spatula.
Spread evenly in the prepared pan and bake for 12 minutes, until it is light brown.
Turn onto a towel dusted with powdered sugar, and roll up the long way inside the towel. This helps it keep its shape once it is filled.
Filling
Once the cake has completely cooled inside the towel whip the heavy cream with the 2T of sugar. Stir in the blueberries.
1 pint heavy cream, 2 Tablespoon sugar, 1/2 cup frozen blueberries
Unroll the sponge cake and spread with the creamy filling. Add extra blueberries if desired. Roll up the cake and set it on a plate.
Frosting
In a mixer whip the heavy cream with 2 T of sugar until soft peaks form. Using a spatula gently fold in the mascarpone, lemon zest and blueberries until just combined.
1 pint heavy cream, 2 Tablespoons sugar, 1 Tablespoon lemon zest, 1/2 cup mascarpone cheese, 1/2 cup frozen blueberries
Spread the frosting over the filled cake roll. Garnish with extra blueberries if desired.