In a stand mixer fitted with a hook, stir together the ice-cold water and salt.

Add the all-purpose flour, bread flour, and softened butter.

Mix until a dough forms.

Knead the dough until smooth, about 2–3 minutes.

Form the dough into a disk, wrap it in plastic wrap, and chill for 1 hour.

Using a stand mixer fitted with a paddle, mix the chilled butter with the Dutch cocoa powder.

Transfer the mixture to a sheet of plastic wrap and shape it into a 7″ x 10″ rectangle.

Wrap in plastic wrap and chill until solid, about 1 hour.

On a lightly floured surface, roll the dough into a 12″ x 18″ rectangle.

Use a pastry brush to dust excess flour from both sides of the dough.

Place the cocoa butter block onto the short end of the rectangle.

Fold the other side of the dough up and over the butter.

Pinch the dough edges to encase the butter.

Working from one short end of the rectangle, use a rolling pin to press into the dough to soften the butter, using a rocking motion between ridges to lengthen the dough.

Roll the dough into a 10″ x 24″ rectangle.

Fold the dough into thirds, like a letter; wrap in plastic wrap and chill for 1 hour.

With one short side of the rectangle facing you, roll the dough into a 10″ x 24″ rectangle.

Fold the dough into thirds, wrap it in plastic wrap, and chill for 30 minutes.

Repeat the rolling, folding, and chilling process 3 more times.

Chill the dough for at least 1 hour before using.

The dough can be frozen for up to 6 months.