Combine the butter, brown sugar, granulated sugar, and vanilla in the bowl of a stand mixer. Using a paddle attachment, beat on medium speed until you have a creamy mixture, about 1 to 2 minutes. Do not beat too much (light and fluffy) or you will incorporate too much air into the dough.

Add the egg and beat until it is thoroughly combined, about 30 seconds.

In a medium bowl, stir together the buckwheat flour, tapioca starch, baking powder, baking soda, and salt. Add the dry ingredients into the mixer and beat on medium speed until the dough comes together.

Add the chocolate chunks and dried cranberries and mix to combine.

Using a 3 tablespoons ice cream scoop, scoop the dough onto a sheet pan lined with parchment paper. Wrap the sheet pan with plastic wrap and refrigerate the cookies for at least 4 hours, preferably overnight.

Preheat oven to 425°F (220°C). Place the oven rack in the middle. Line a sheet pan with parchment paper. Place half of the refrigerated cookies on the prepared sheet pan leaving at least 2 inches (5 cm) in between cookies. Sprinkle the top with flaky sea salt, if using.

Bake the cookies for 8 to 10 minutes until they are golden brown, but the center still appears underbaked. They will have puffed up slightly.

Remove the pan from the oven and tap it on your counter or stove 3 times to deflate the cookies. If you want the cookies to be perfectly round, place a large cookie cutter over them and swirl it around. Do this right after the cookies come out of the oven while they are still very soft and pliable.

Let the cookies cool on the pan for 5 minutes. Serve warm or at room temperature. Finish baking the cookies. Store them in an airtight container, at room temperature, for up to 5 days.