To make the dough, put the flour, salt, sugar and butter in a large bowl and rub together till the mixture resembles breadcrumbs. Add in the zest, yeast, red food colouring and warm milk and mix to a soft dough. Place the dough onto a floured surface and knead well for 4 minutes. Put the dough into a floured bowl, cover with plastic and leave to rise till double in size, for about an hour. Knock back the dough and roll out to a large square, 40cm x 25cm. Brush the dough with half of the melted butter, then evenly spread out the cold custard using a palette knife. Sprinkle over the cinnamon and white chocolate chips. Roll up tight from top to bottom, into a large Swiss roll and cut up into 12 equal slices. Brush a rectangular baking tray with some of the remaining butter, and place the slices on their sides on the tray, 3 pieces by 4 pieces. Brush with the remaining butter. Cover with plastic and leave to rise again till double in size again. Bake the buns in the oven for 35-40 minutes at 210`c, till golden brown. Meanwhile, mix the icing sugar with a little warm water to make a glace icing. As soon as the buns come out of the oven, brush them all over with the icing to glaze. Finally, cover the icing whilst still soft with pink sprinkles and serve immediately.