Place the milk, water, yeast, and sugar in a measuring cup or similar container and stir until the yeast has dissolved. Stir in a tablespoon of flour and let the pre-dough rise, covered, for 10 to 15 minutes, until bubbles have formed on the surface.

Place the remaining flour and salt in a mixing bowl. Slowly pour in the yeast mixture and stir it into the flour. Add the butter and egg, and knead the dough in a stand mixer for about 10 minutes until smooth. Alternatively, knead the ingredients by hand for 10 to 15 minutes. Shape the dough into a ball, return it to the bowl, cover it, and let it rise in a warm place for one hour.

Place the dough on a floured work surface, shape it into a ball again, and divide it into six equal pieces. Shape each piece into a ball, then flatten it into a relatively thin patty, and place it on a baking sheet lined with parchment paper. Cover the baking sheet with a clean tea towel and let the rolls rise for another 30 minutes.

Preheat the oven to 180°C (350°F) convection or fan-forced (200°C (400°F) conventional). Whisk the egg yolk with the milk and brush the rolls with the mixture, then sprinkle with poppy seeds or sesame seeds. Bake the burger buns for 18 to 20 minutes. Let them cool on a wire rack.