Whizz the mint leaves into the sugar in a food processor until very finely chopped. Line and butter, two x 18cm sandwich tins. Heat the oven to 180C.
Whisk the eggs and mint sugar together until the mixture is thick enough to hold a trail. It should look like a foam of very small bubbles, which will take up to 10 minutes. Sift over some of the flour and fold it in carefully, then add a little more and some of the butter. Keep folding in the flour and butter until it is all incorporated. Be careful not to squash out the air bubbles. Carefully scoop the mixture into the tins. Bake for 20-25 minutes or until the cakes are risen, lightly browned and spring back when pressed lightly. Cool in the tin for 5 minutes then turn out onto a rack.
Spread the base layer with some strawberry jam and layer on some whipped cream followed by some sliced strawberries. Put the top on the cake. Mix the icing sugar with enough water to make a glacé icing (flavour with the mint if you like, using a drop at a time) that you can spoon over the cake. Run the icing back and forth across the cake to make a cross-hatch pattern and leave to set. Add some strawberries and mint leaves to decorate.