Warm 350ml water in a saucepan until steaming, then remove from the heat, add the butter and set aside to cool until you can comfortably put your finger in the liquid. Combine the flour, yeast, sugar and 1 tsp salt in a large bowl or the bowl of a tabletop mixer. Add the cooled liquid and combine to make a soft dough. Knead for 10 mins by hand, or 5 mins in a mixer, until the dough feels stretchy and smooth. Return to a clean, oiled bowl and cover with oiled cling film. Leave somewhere warm to rise for 11/2-2 hrs or until doubled in size.
Grease and line your largest baking tray (ours was 42 x 33cm) with baking parchment. Grease the parchment too, then scatter over the polenta. Remove the cheeses from their boxes and place on the tray, spaced well apart. Brush each one with a little oil, slash the tops a few times with a sharp knife and poke in some fresh herbs, if you like.
Pour the egg into a shallow dish, the poppy seeds in another and the sesame in a third. Knock the air out of the dough and knead again for several mins. Take a small piece of dough (about the size of a whole walnut ) and roll it into a ball. Dip the top in egg, then into one of the dishes of seeds, and place on the baking tray, seeded-side up. Continue rolling and dunking the dough, leaving every third piece seedless, filling the tray but leaving a little space between each ball of dough. When you have used all the dough, you may need to reposition the dough balls to ensure the baking tray is evenly covered.
Heat oven to 180C/160C fan/ gas 4. Cover the tray with a few sheets of oiled cling film and leave for 30 mins-1 hr until the dough has doubled in size and the balls are touching. Bake for 25-30 mins until the dough balls are cooked through and the cheese is oozy and melted. Leave to cool for 5 mins, then serve.