1. In a medium saucepan over medium heat, melt the butter with the dry milk powder, stirring and swirling the pan often, until the butter foams, turns golden brown and smells nutty, about 5 to 7 minutes.
Pour into a large bowl and set aside to cool, about 15 minutes.
2. In a medium bowl, whisk together the plain flour, buckwheat flour, baking soda, and salt.
Line 3 large baking sheets with greaseproof/parchment paper, if baking all at once.
Tip: If baking in batches, line one quarter sheet pan with parchment and pull out a half sheet pan to cool or freeze the rest.
3. Add the sugars to the melted brown butter; mix until combined.
Add the egg, egg yolk and vanilla extract and stir until combined. Add the flour mixture and stir until well combined.
4. Stir in the chocolate.
Working one at a time, scoop out about 82 grams of the dough and roll into a ball. Place the balls on the prepared baking sheets, and chill for 1 hour and up to 24 hours.
Tip: If you don’t want to bake all of these at once, put some of the rolled dough balls on a covered baking sheet in the freezer until frozen. Then, remove the baking sheet and store the cookie dough in a freezer-safe bag or container until ready to eat. They can be baked directly from the freezer, you may just need to bake for a few extra minutes.
5. When ready to bake, heat Ooni Volt to 175 °C degrees.
Bake just until the edges start to turn golden, rotating halfway through the bake at about 7.5 minutes, for a total bake of about 15 minutes.
6. Remove from the oven and sprinkle cookies with flaky sea salt to taste.
Put the baking sheets on a cooling rack and let sit for about 10 to 15 minutes before enjoying. Store cookies in an airtight container at room temperature for up to 3 days.
