Dice the butter. The trick here is to dice the butter small, into 1/4" cubes. Place the diced butter in a bowl and transfer to the freezer while you're getting the rest of the ingredients together.
Mix the dry ingredients. In the bowl of a stand mixer, whisk together the flour, sugar, salt, baking powder, dehydrated onions, and baking soda. Mix in the chives and cheddar.
Mix in the butter. Remove the butter from the freezer and add it to the bowl of dry ingredients and toss well. Mix with the paddle attachment on medium-low until the mixture resembles clumpy sand with pea-sized pieces of butter.
Mix in the wet ingredients. With the mixer running, stream in the buttermilk and mix just until the dough clumps together. Transfer to a lightly floured work surface and press the dough into a 8x11 rectangle.
Laminate the dough. Fold the rectangle into thirds and press into a 1-inch thick rectangle. Repeat folding and pressing 3 more times. Press the dough into a 9" square that's about 1" thick.
Preheat an oven to 450 F.
Cut and bake. Using a sharp knife, cut into 9 equal squares and transfer the biscuits to a parchment-lined baking sheet and refrigerate for 30 minutes. Brush the tops with the remaining buttermilk and sprinkle with flaky salt. Bake for 18-20 minutes, until puffy and golden brown.