Gently warm the milk until it is just about lukewarm, then crumble in the yeast (if using dried yeast, skip this part and add the yeast to the dry ingredients later). Let the yeast activate for around ten minutes (until it starts to froth).
Melt the butter and leave to cool for around five minutes, then add to the yeast liquid. Beat in two eggs and stir until everything is well combined.
In a separate bowl, add around 350g (3 cups) flour, the sugars, vanilla sugar and pinch of salt, and mix to combine.
Pour the liquid into the dry ingredients and stir quickly with a spoon. Gradually add more flour as necessary until you have a workable dough (very sticky but not runny).
Knead until the dough becomes elastic, again only adding a touch of flour when it is too sticky to knead. Pop back into the bowl, cover and leave in a warm place for around 45-60 minutes until the dough has pretty much doubled in size.
On a floured surface, knock the dough back and knead again until it is once more nice and elastic and then roll out into a large rectangle shape (aim for about 45cm x 30cm as a rough guide). Melt the butter and then brush it all over the dough (don't go right to the edges but leave about 1/2 inch all the way around). Sprinkle with your dried fruit and zest (if you are using it) and then with the shorter side in front of you carefully roll it up. For most buns I make, I start with the long edge in front of me, but to me, Belgian buns are fairly flat and very large, so I wanted to try and reproduce that rather than a cinnamon bun, for example.
Line or grease two large baking trays and pre-heat the oven to 190°C (375°F). Carefully cut the dough into thin slices and place them on the baking tray with plenty of space apart (they do spread out). Bake for around 15-20 minutes, just until golden (do not overbake - they are ready when the bottoms sound hollow when tapped. Too much cooking will result in dry buns).
Remove them from the oven, pop onto a wire rack, and allow to cool completely. In the meantime, prepare the icing - start of with around 1 cup (100g) icing sugar and add just enough milk to produce a very thick icing (it should spread over the buns gradually, not drip right off). Sprinkle with a little lemon zest if using, top with your glace cherry (if you have to), and then get those beauties scoffed. Enjoy!