1. Preheat oven to 375. Line a baking sheet with parchment.
2. Grate cold butter and place in freezer to firm up.
3. In a medium bowl whisk together flour, sugar, baking powder, baking soda and salt.
4. Take half of frozen butter and work into flour mixture pretty well. The goal is to coat a lot of flour with fat to help with tenderness. Then add remaining butter and cut in quickly, leaving some pea sized pieces.
5. Add buttermilk in 2 batches (you may not need all of it), mixing in gently with a rubber spatula. Turn out onto a lightly floured work surface. Dust hands with flour and gather up dough. Press gently to create a thick rectangle. Perform 4 folds, then neaten up the rectangle and use a knife to gently cut off folded edges (to open up layers on sides for expanding in the oven). Cut into 6 equal squares. Place on parchment lined sheet. Brush with additional buttermilk and sprinkle with fleur de sel. Bake until light golden, about 15-17 minutes.
*You can freeze your unbaked, shaped biscuits for 30 minutes before baking for extra flakiness, and you can also store the unbaked biscuits frozen for up to 1 month for fresh baked biscuits at a moment's notice, just know you'll need to increase the baking time slightly when baking from frozen.