Mix everything together in your mixer until you get a nice really smooth dough. This can take a while. In my case it took about 15 minutes, scraping down the dough every few minutes from the dough hook in the mixer.
After it's mixed, place the dough in a lightly oiled bowl, turn to coat (so it won't dry out) then let it rise in a draft-free place until doubled. This might take as much as 2 hours since the large amount of butter will slow the rise. WAIT!
After rising, de-gas the dough, cut it into about 12 equal pieces, press them fairly flat then add some chocolate chips.
Roll the dough into a round shape and place on parchment paper to rise, covered for about 45 minutes. If you want, you can instead place the filled dough balls in a lightly greased muffin tin to maintain the round shape. They do come out much prettier that way! Let the dough rise in the tin until they overflow the tops a little to make the traditional 'mushroom' top.
Pre-heat the oven to 365 F (185 C). Just before placing in the oven, brush gently with a wash of egg white and water. Sprinkle with sesame seed if you want. I think it make them tastier but it's just a matter of taste. Bake for 10 minutes, turn them 180 degrees for even baking, them bake another 10 minutes.