Orange Cake with Blueberry Cream Frosting

Directions

1. Put the blueberries in a bowl, sprinkle with 2/3 cup of sugar and leave to thaw at room temperature.
2. Preheat the oven to 350°F. Grease and line a 12 x 15 inch sheet cake pan.
3. In a large, clean bowl, beat the egg whites and salt until soft peaks form. In a separate bowl, beat butter and 1 3/4 cups sugar until pale and fluffy. Beat in the egg yolks, one at a time, followed by 2/3 cup of yogurt. Sift the flour and baking powder into the bowl and stir in gently. Add a quarter of the egg whites and fold in with a metal spoon, then fold in the remainder. Pour into the pan and level the surface. Bake for 25-30 minutes or until just firm to the touch.
4. Meanwhile, combine orange zest, orange juice and remaining sugar in a saucepan. Heat gently until the sugar dissolves. Bring to a boil and boil for 6-8 minutes until syrupy. Spread the syrup over the cake when removed from the oven. Leave to cool.
5. Soak the gelatin according to package instructions. Bring 2/3 cup of the cream almost to a boil in a small saucepan. Add the gelatin and stir until dissolved. Crush the blueberries with a fork and stir in the remaining yogurt. Whip the remaining cream until firm peaks form. Stir the gelatin mixture into the blueberry mixture, then fold in the whipped cream. Spread over the cake and leave to set for several hours or overnight.
6. Cut into squares and serve sprinkled with pistachios.