Pre-heat oven 325°F (165°C). Sift together the flour and icing sugar in a bowl. Cut in the butter and add the lemon zest. Knead until blended. Form dough into ¾-inch (2cm) balls. Press evenly into 1 ½-inch tart cups on the bottom and up the sides to form a shell. Bake for about 18-20 minutes or until golden brown. Let cool and remove from cups and place on wire rack to cool.
Combine all ingredients and bring to a boil. Simmer gently for 10 minutes, stirring constantly. Mash the dates with a fork as they cook. Let cool and spoon into tart shells.
Cream the butter and gradually add in the icing sugar. Add the milk, maple syrup and remaining sugar. Beat at high speed until smooth. Icing should be thick enough to pipe. Pipe a swirl of icing on top of the filled tarts.