In a medium bowl, add the active starter and water. The starter should be floating. Combine the two and slowly add the flour. Mix everything until it just comes together. Set it aside for 4 hours at room temperature or until the levain has doubled in size.
30 g active sourdough starter, 120 g all-purpose flour, 110 g water
Make the brioche dough
In a large bowl, combine the levain with milk, whisked eggs, maple syrup, and vanilla essence.
1/2 cup milk, 1 tsp vanilla essence, 3 eggs
Slowly add the flour and salt and combine everything to a homogenous dough.
4 cups all-purpose flour, 1/4 cup maple syrup, 1 tsp salt
Cube the butter with a knife and fold it gently into the dough. Knead the dough for about 10 minutes until it passes the window pane test.
1/3 cup butter
Cover the bowl with a damp tea towel and chill it in the fridge for 1 hour. Don't skip this step as you won't be able to shape the wet dough otherwise.
Remove the dough from the bowl and divide it into 8 equal-sized pieces. Shape the pieces into seamless balls and place them side by side (2x4) into a greased loaf tin. You need to shape them quickly or the dough will get too soft to work with.
Cover the tin with a damp tea towel and let it proof on your kitchen bench for 8-10 hours (overnight works well) until they're double in size.
Preheat the oven to 356F/180C.
Combine the milk and egg yolk and brush the top of the brioche with the glaze. Bake it for 20-25 minutes until golden brown.