Stir the yeast and a pinch of the sugar into the warm milk and let stand for 8 minutes until foamy.
Mix the flour, remaining sugar, butter, egg yolks, salt, lemon zest, and yeast mixture. Knead for 8 to 10 minutes until smooth and elastic.
Cover and let rise for about 75 minutes until doubled.
Prepare the Fillings:
Mix the farmer cheese with 2 tablespoons sugar, one yolk, and vanilla.
Simmer the ground poppy seeds with 1/2 cup milk and 1/4 cup sugar for 5 minutes, stirring, then cool.
Rub the streusel flour, sugar, and cold butter together into irregular crumbs.
Shape and Bake:
Divide the dough into 18 pieces, shape into rounds, and place on parchment-lined trays. Cover and rise for 30 minutes.
Press a deep wide indentation into each round. Fill with cheese, poppy seed mixture, or plum butter; scatter with streusel and brush the exposed dough with egg wash.
Bake at 375 F (190 C) for 15 to 18 minutes until broadly golden at the edges. Cool on the trays for 5 minutes, then transfer to a rack.
Notes
Use a thick plum butter rather than loose jam so the filling stays in the center.
Well-drained ricotta works when Czech tvaroh or farmer cheese is unavailable.