Mix yeast with warm water and a pinch of sugar, set aside for 10-15 minutes in a warm place (by then the yeast should be foamy). Sift flour into a large bowl, add the foamy yeast, sugar, salt, and 3 eggs. Mix the ingredients with a wooden spoon. Gradually add the remaining 2 eggs while working the dough (by hand or with a mixer fitted with a dough hook). Work for about 10-15 minutes until the dough is smooth and elastic.
To the worked dough, gradually add butter, 2 tablespoons at a time, constantly mixing or kneading. Add the next pieces of butter quickly, as soon as the previous portions blend in with the dough. Do not work it too long, just until the butter blends with the dough.
Cover the bowl with transparent film and set aside in a warm place until it doubles in volume, about 1 and 1/2 hours. Punch the dough with your fist, form a ball, cover again, and place in the refrigerator overnight (the dough can stay in the refrigerator for a maximum of 3 days).
Remove dough from refrigerator, knead on the work surface, and cut into 16 equal pieces. Roll into patties (about 6-7 mm thick), dusting with a little flour. Spread with jam and roll into a roll. Fold each roll in half and cut with a sharp knife from the fold side to 3/4 of the length. Unfold the dough to create a heart shape. Arrange on two baking sheets lined with parchment paper.
Loosely cover with film and set aside to rise for about 1 and 1/2-2 hours.
Preheat oven to 190°C. Brush the top of the buns with beaten egg. Place the first baking sheet in the middle section and bake until golden for about 15 minutes (warning! brioche quickly begins to brown). Cool on a metal rack. Sprinkle with coarse sugar. Buns keep fresh for a long time. I keep them in a closed container. I always reheat them! They are best when warm.
