In a double boiler, melt 100 gms butter and 100 gms castor sugar. Add some lemon zest, lemon juice and 2 egg yolks and mix well. Pour the mixture to another pan and cook on a low flame for 10 minutes. Cool the mixture and pour the lemon curd in a piping bag. Pipe the curd on some tarts and garnish with mint leaves. Lemon Curd Tart is ready.