To make the brioche dough, put the flour, sugar, salt and yeast into the bowl of an electric stand mixer and briefly mix together to combine. With the dough hook attached, add the milk, egg and egg yolk, and mix together on a low/medium speed until a shaggy dough is formed. Continue kneading in the machine for 10-15 minutes or until the dough is smooth and elastic. Add the butter a couple of pieces at a time, adding more once the previous pieces have been combined into the dough. Once all the butter has combined, knead the dough for another 10 minutes or until smooth and elastic.