Sprinkle the white chocolate over a baking sheet and bake in the center of the oven for 10 minutes. Take out of the oven and stir the chips and place back in the oven for 10 more minutes. Repeat this process 2-4 more times, until they have turned a nice bronze color and the chips have crumbled. Take out of the oven and set aside to cool.

In a medium bowl whisk together the bread flour, all-purpose flour, cocoa powder, baking powder and sea salt. Add this to the butter mixture and combine in as few strokes as possible and no flour streaks remain. Take the bowl out of the mixer, sprinkle the caramelized chocolate dust over the top and barely fold into the dough…you want there to be thick swirls of it in the dough. Wrap tightly and chill for at least 3 hours or overnight.

Preheat the oven to 350°F (177°C) and cover several baking sheets with parchment paper.

Roll into 1 tablespoon-size balls (19g) and set on the prepared baking sheet leaving a 1 1⁄2-inch (3.7-cm) space between dough balls. If dough gets sticky while rolling, place it in the freezer for 10 minutes to firm back up.

Bake one sheet at a time for 8 minutes in the center of the oven. Let cool for 5 minutes on the baking sheet and then transfer to a rack to finish cooling.

Using one hand to whisk constantly, pour half of the hot milk mixture into the egg mixture. This helps to temper the eggs and stop them from scrambling. Whisk until incorporated, and then pour the whole lot back into the saucepan.

Heat the milk and egg mixture over medium heat, whisking constantly, until it begins to bubble. It will thicken quickly. Once it has thickened, cook for one minute, then remove from the heat. Pour into a shallow container or bowl of a stand mixer and press some plastic wrap over the surface to avoid a skin from forming. Refrigerate until cold - at least four hours, preferably overnight. If you need to speed this process up, you can place the pastry cream in a bowl, then place the bowl in an ice bath. Stir frequently.

Fit your mixer with the whisk attachment, and place the pastry cream in the bowl. Whip the mixture on medium until creamy and lump-free. Begin adding the butter, a few cubes at a time, until fully incorporated. It may look curdled at some point but just keep whipping - it will come together! Once the buttercream is homogenous, add the cooled melted dark chocolate and flaky sea salt, and mix well to combine. Store in an airtight container until ready to use, or if using immediately, transfer to a piping bag fitted with a round tip.