The night before, in a bowl, mix together the flour, sugar, eggs, yogurt, salt, and yeast until the mixture is well combined.
Cover the bowl with plastic wrap and refrigerate overnight.
The next day:
Roll out the dough into a rectangle 3 times longer than it is wide.
Spread the butter over 2/3 of the dough and fold it into 3 (the unbuttered part in the middle, then the last 1/3 on top).
Turn the dough 1/4 turn and roll it out again.
Repeat this process 3 times.
Roll out the dough on a floured surface into a large rectangle of 18″ x 13″ x 1/4″ (35 cm x 30.5 cm x 0.5 cm).
Mix the softened butter, brown sugar, and cinnamon with a fork and spread the mixture over the dough, leaving a 1/2″ (1 cm) border around the edges.
Roll the dough into a log, squeezing it slightly.
Cut the log into 12 slices of 1″ (2.5 cm) wide.
Place the slices in 12 greased muffin tin cups.
Brush with a little beaten egg mixed with a little milk and let it rise at room temperature for about 2 hours.
Preheat the oven to 350 F (180 C).
Brush with egg again and bake the brioches for 20 minutes or until they are golden brown.
Let them cool on a wire rack.