For the roulade sponge
Pre-heat the oven to 180°C.
Line a baking tray with a sheet of greaseproof paper.
Using a handheld electric whisk or stand mixer, whisk together the eggs and sugar for 3-4 minutes or until it reaches the thick fluffy “ribbon” stage.
Fold in the flour, ensuring it is evenly mixed without losing too much of the volume. Pour the mixture into the tray lined with greaseproof paper and bake in the oven for 10-12 minutes.
Once the sponge is cooked, remove from the oven and leave to cool.
Once cooled, place a clean tea towel onto the work surface and turn your sponge out onto the towel. Carefully peel back and discard the greaseproof paper. Brush evenly with the boiled kirsch if you are using it.
For the filling
In a large bowl, fold together the mascarpone or crème fraîche with the Saint Agur blue cheese crème and the icing sugar until evenly mixed.
Fold in half of the blueberry purée, but do not entirely mix together, giving you a contrast of white and purple/blue swirls.
Reserve the other half of the blueberry purée to serve with the sliced roulade.
To create the roulade
Using a palette knife, spread the blueberry and Saint Agur filling evenly over the sponge, making sure you leave a 1cm gap from the longer edges and leave a gap of 5cm from one end of the shortest side.
Arrange the sponge so that the shortest side is facing you with the 5cm gap of filling facing away from you.
Using the tea towel for support, carefully but firmly roll the roulade up making sure you don’t press too hard and squeeze the filling out.
If you wish, roll the roulade in a little caster sugar or simply dust with icing sugar, or alternatively cinnamon powder mixed with caster sugar.
Trim the edges of the roulade with a serrated knife before cutting into 6-8 generous slices. Slice and serve with a spoon of the blueberry purée.