Create the sponge.
In a large bowl, dissolve the yeast in the warm water.
Stir in 1 tablespoon of the sugar and about 2 cups or 240g (8.5 oz) of the flour.
Mix until you have a soft, spongy batter.
Cover the bowl with a towel and set it in a warm place to rise for about 45 minutes, until it's puffed up and bubbly.
Mix the dough.
Add the milk, the remaining 1 tablespoon (15 ml / 0.5 fl oz) of sugar, the salt, and the melted butter to the sponge and stir well to combine. Gradually work in the remaining flour, about a cup at a time, until a fairly stiff dough forms that pulls away from the sides of the bowl.
Knead to perfection.
Turn the dough out onto a well-floured surface.
Knead for 10 to 15 minutes, adding small amounts of flour as needed to prevent sticking, until the dough is smooth, supple, and elastic.
First rise.
Form the dough into a ball and place it in a large, buttered bowl, turning it once to coat all sides.
Cover with a light towel and let it rise in a warm, draft-free spot for 45 to 60 minutes, or until it has doubled in size.
Shape the loaves. Gently punch down the dough to release the air.
Divide it in half and shape each portion into an oblong loaf or a large braid.
Place the loaves on buttered baking sheets.
Second rise.
Cover the loaves with a light towel and let them rise in a warm place for about 30 minutes, or until they have nearly doubled in size.
While they rise, preheat your oven to 425°F or 220°C (425°F).
Glaze and bake.
In a small bowl, beat the egg yolk and 1 tablespoon (15 ml / 0.5 fl oz) of milk together. Gently brush this glaze over the tops of the loaves.
Place them in the preheated oven and bake for 5 minutes, then lower the heat to 375°F or 190°C (375°F).
Continue baking for another 35 to 40 minutes, until the loaves are a deep golden brown and sound hollow when tapped on the bottom.
Cool completely.
Transfer the hot loaves to a wire rack to cool completely before slicing. This step is crucial for the bread's final texture, so be patient!