For more detailed steps for the brioche dough process, it can be found here Chill everything: Weigh all dough ingredients and refrigerate overnight or at least a few hours ahead — including eggs, milk, butter, and your mixing bowl.
Initial mix: Combine bread flour, T65 flour, sugar, salt, yeast, eggs, and cold milk in the cold mixing bowl. Mix on low until no dry flour remains then increase to medium and knead until cohesive and developing strength.
Check dough temp: Should be around 68°F before butter goes in. If too warm press plastic wrap on surface and refrigerate entire bowl until back to 68°F.
Add butter: Lower speed slightly. Add cold butter in thirds, waiting for full incorporation before the next addition. Alternate low and medium speed. Temperature must not exceed 75°F.
Windowpane test: Dough should be smooth and glossy with a thin translucent windowpane. Final temp should not exceed 75°F.
Bulk ferment: Shape into a smooth ball, cover, and ferment at room temperature for 1 hour.
Letter folds and cold ferment: Degas on a lightly floured surface and perform 2 rough letter folds. Place in an airtight container, flatten slightly, and refrigerate at least 12 hours and up to 24 hours.