In a large mixing bowl or a stand mixer, combine all-purpose flour, sugar, and salt. Crumble in fresh yeast, mixing it through. Add beaten eggs and incorporate softened unsalted butter. Knead for about 20 minutes until smooth and slightly sticky, then form into a ball.

Place the brioche dough in a greased bowl, cover with a damp cloth, and let it rise in a warm area for approximately 2 hours, until it has doubled in size.

While the dough rises, make the custard filling by warming whole milk and unsalted butter in a saucepan over medium heat with the seeds from a vanilla pod. In a separate bowl, whisk egg yolks, sugar, and cornstarch until smooth. Add the warm milk mixture, then return to the stove and stir until thickened, about 5 minutes. Let cool.

After the first rise, turn the dough onto a floured surface and roll into a rectangle. Spread cooled custard over the surface, sprinkle with mini chocolate chips, fold like a letter, cut into small pieces, and place in a greased pan.

Cover the brioches with a kitchen towel and let rise for about 2 hours in a warm environment.

Preheat oven to 350°F (180°C). Beat egg and egg yolk, brush over the tops of the brioches. Bake for 10-12 minutes until golden.

Prepare a sugar syrup by combining water and sugar in a saucepan, heating until the sugar dissolves. Brush syrup over baked brioches for a glossy finish.

Let the brioches cool slightly before serving. This ensures perfect texture when cutting into them.