Place the flour in a bowl and make a well in the centre.
Crumble the yeast into the well. Pour in 3tsp lukewarm water, add the sugar and mix with a little flour. Let the pre-dough swell, covered, for 15 minutes.
Add the egg, butter and salt to the pre-dough, and knead everything with 200ml lukewarm water to a smooth dough. Cover and leave to rise for 45 minutes.
Preheat the oven to 180°C (160°C fan, gas 4). For the filling, crush the feta with a fork and mix it with 75g butter. Knead the dough on a floured surface once more and divide into 4 portions. Flatten each portion into a circle about 3mm thick, and spread with some feta butter. Roll in the dough.
Cut the roll slightly diagonally, once to the right, once to the left, into pieces.
Melt the butter over low heat and greasae the tin with it. Roll out the remaining dough, fill it in the same way and cut into pieces.
Place the dough pieces side by side in the baking tin.
Cover and leave to rest again for 30 minutes.
Mix the egg yolk with the remaining butter and brush over the dough. Bake in the oven for 40 minutes until golden brown.