Preheat the oven to 180C, gas mark 4. Grease and base line a 20cm cake tin.
Cream the butter and sugar together until pale and fluffy. Whisk in the eggs, one at a time. Mix the flour and baking powder together and fold into the butter mixture.
Stir in the carrots and walnuts, reserving 2 tbsp walnuts for decorating. Spoon into the cake tin and bake for 1-1¼ hours or until golden and a skewer comes out clean.
Remove from the tin and allow to cool.
Mix the soft cheese with the icing sugar and spread over the top of the cake. Decorate with sprigs of fresh, living mint and sprinkle over the reserved walnuts.