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 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, chocolate chips and pecan nuts. 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, finish off the buns with a generous sprinkling of the extra cinnamon and serve immediately.