In a medium bowl, whisk together the flour, two types of cocoa, bicarbonate of soda and both types of salt.
In the bowl of a stand mixer fitted with the paddle attachment, combine the butter and sugar. Beat on medium speed until light and fluffy - about 2 to 3 minutes. Use a rubber spatula to scrape the sides and bottom of the bowl as necessary.
With the mixer on low, gradually add the dry ingredients. Increase to medium-low and beat the dough until the dough clumps around the paddle and/or the sides of the bowl - about 2 to 3 minutes.
Add all the white chocolate at once and beat on medium low until the chocolate is evenly distributed throughout - about 1 minute.
Tip the dough onto the counter and used your hands to shape it into a roughly 6-inch (15 cm) square. Use a bench scraper to divide the dough into 4 equal portions. Place one portion in the centre of a piece of plastic wrap and use your hands to shape it into a rough log. Fold the plastic over the log so it covers the dough completely and use your hands to roll the dough as you would clay, rolling it back and forth in quick motions until it forms a circular log about 1 1/2 inches (~4cm) wide and 5 inches (~13cm) long. Seal the plastic wrap tightly around the dough. Repeat with the remaining dough portions and refrigerate them or overnight (see bottom of recipe if you want to bake on same day OR bake later)