240ml lukewarm water
2 teaspoons fine sea salt
284g unsalted butter, preferably cultured, softened
1 large egg, beaten with 1 tablespoon of water for the egg wash
Method
In the bowl of a stand mixer fitted with the paddle attachment, combine both flours, the sugar and yeast. In a separate bowl, vigorously whisk the water, milk and salt until the salt dissolves. Add the mixture to the flour and mix on medium-low just until the dough comes together.
Switch to the dough hook (or, if you’re kneading by hand, transfer to a lightly floured work surface). Increase the speed to medium-high and knead until the dough is smooth and somewhat elastic, about five minutes. Transfer the dough to a clean bowl, cover with a towel or plastic wrap, and refrigerate for at least six hours, and up to overnight.