Grease the cake tin with a little butter and sprinkle with 1tbsp of flour. Tap the tin, and turn it to coat all sides.
Boil the milk for the filling and put to one side. Whisk the egg, sugar, lemon and orange zest in a pan, put it on very low heat, and warm through, whisking all the time. Remove from the heat and stir in the flour. Return to very low heat and whisk in the hot milk. When