Preheat an oven to 190 degrees. Line a baking tin with baking parchment 15cm x 20cm. Add a pinch of sugar to the matcha powder and mix well. Sift the sarrasin and corn starch together. Warm the milk and the butter together in a small bowl, then add to the matcha little by little. Mix until the matcha power is well dissolved. Set a side.

Beat the egg yolk and 25g sugar with whisk in a big bowl until the colour changes to white and it it becomes fluffy. Set a side. Beat the egg white with a whisk until soft peaks form, then add the sugar a third at a time. Whisk to a thick foam.

Add 1/3 of an egg white to the egg yolk, mix with a whisk. Then change to a spatula, add another 1/3 of egg white, then the final 1/3. When it's almost mixed, add the sifted sarrasin and starch mix. Try not to make a any lumps. Then add the matcha milk and butter mix. Mix well (about 60 times). Pour into the cake tin, and bake for 13 minutes.

Check the sponge with a toothpick: if it doens't stick, it's ready to take out of the oven. Put the sponge onto the cake cooling grid for 2 minutes, then turn it over for 2 minutes. Repeat, then keep it under cover until use.

Add the sugar to the cream and whisk into soft peaks. Spread the cream evenly over the sponge, then roll it to make your Swiss roll. Then warp with food cling film. Keep it in a refrigerator for at least 2 hours before serving.