Knead together the butter, rikotta, zests and flour into a soft dough. Transfer onto a lightly dusted surface and roll into 1cm thick dough and cut into 8 cm squares. Place a teaspoon of the apricot jam in the centre of the obtained square and a slice of apricot. Flip the pastry over to form a triangle. Pinch edges with finger tips to secure the filling. Place triangles onto a greased baking tray and brush with egg white and sprinkle with the caster sugar. Bake in a hot oven temp 180°c for 15 to 20 minutes until pastries are golden brown.