Brioche dough
In the stand mixer bowl, dissolve the yeast in the buttermilk with the two pinches of sugar.
Let it sit for 10 minutes or until the mixture foams.
Add the melted butter, egg yolk, and vanilla seeds and mix.
Add the flour, sugar, and salt.
Mix on low speed with the hook for 6 minutes.
Add a little flour if the mixture is too sticky.
Form a ball with the dough and place it in an oiled bowl, turning it to coat it evenly with oil.
Cover and let it rise until it has doubled in volume, about 1 hour and 15 minutes.
When the dough has doubled, place it on a floured surface and roll it out into a 12 x 14 inches (30 cm x 35 cm) rectangle, with the long side facing you.
Mix the softened butter, brown sugar, and cinnamon with a fork and spread the mixture over the dough, leaving a 1/2 inch (1 cm) border on the side facing you.
Brush this border with a little water.
Roll the dough and seal the edges well.
Cut the roll into 12 portions.
Butter a 9 x 13 inches (23 cm x 33 cm) mold and place the brioche rolls, leaving a little space between each.
Cover and let them rise for about 40 minutes.
Preheat the oven to 325 F (160 C).
Bake the brioche rolls for about 30-35 minutes, or until they are golden brown.
Let them cool on a wire rack.
The glaze:
Mix the powdered sugar, cream, and softened butter together until smooth.
Spread over the brioche rolls and enjoy.