Crumble the yeast and mix into the lukewarm water. Put the flour and yeast water in a large enough bowl and mix thoroughly. Add the butter and knead into the dough, seasoning with salt in between. Let the dough rise at room temperature for 3/4 hour with the lid closed.
Divide the dough into 10 on the spot pieces, roll into sausages and then shape into rings. Using a skimmer, dip the rings one at a time into the boiling water for 1/2 minute each and shape onto a baking sheet (it is best to place baking parchment underneath). Brush with egg yolk and sprinkle with sesame seeds. Turn on the oven to 175 degrees Celsius and bake the sesame rings in half an hour until golden brown.