Preheat the oven to 160˚C. Line and grease a 6-inch tart base or wider if preferred.
In a bowl, combine the butter and caster sugar until light and fluffy.
Stir in the egg and egg white.
Add the plain flour and baking powder.
Stir in the milk and combine thoroughly and flavour strawberry jam and lemon zest.
Spread the mixture in the tin then brush the top with egg wash.
Place in the oven and bake for 30-35 minutes in the oven.
Remove and leave to cool for 10 minutes before serving with some whipped cream.