1. Make the water roux: In a small saucepan, whisk together 25g bread flour and 125ml water. Cook over medium heat, whisking constantly, until the mixture thickens and becomes a paste. Remove from heat and let it cool.
2. In a mixing bowl, combine the cooled water roux, 350g bread flour, 55g sugar, 5g salt, and 7g instant yeast.
3. Add 125ml milk and 1 egg to the dry ingredients. Mix until a dough forms.
4. Knead the dough until it becomes smooth and elastic. Add 30g softened unsalted butter and continue kneading until the butter is fully incorporated and the dough is smooth.
5. Place the dough in a greased bowl, cover, and let it rise until doubled in size, about 1 hour.
6. Punch down the dough and shape it into a loaf. Place it in a greased loaf pan. Cover and let it rise again until doubled in size, about 30 minutes.
7. Preheat the oven to 350°F (175°C). Bake the bread for 30 minutes, or until golden brown and the loaf sounds hollow when tapped.
8. Remove the bread from the oven and let it cool in the pan for 10 minutes. Transfer to a wire rack to cool completely before slicing.