Preheat oven to 300°F. Spread the baking soda on a foil-lined sheet pan and set aside.
In the bowl of a stand mixer with a dough hook, combine flour, yeast, brown sugar, and butter.In a separate bowl or measuring cup, whisk the salt into the water until dissolved.
With the mixer running on low, pour the salt water into the mixer bowl. Increase the speed slightly to give the dough time to come together in a shaggy mass, pausing occasionally to scrape down the sides of the bowl.If the dough seems really dry (lots of dried clumps of flour in the bottom of the bowl that aren't being picked up by the dough hook) you may need to add water ½ tablespoon at a time, giving it time to incorporate before adding more. Resist adding more water unless it seems absolutely necessary.
Once the dough has just about come together, increase the speed of your mixer and knead the dough for 2-4 minutes until it's smooth, supple, and just slightly tacky (but not sticky) to the touch. If the dough sticks to the sides of the bowl, dust in more flour 1 TSBP at a time until the dough fully pulls away from the sides. Shape the dough into a ball, place into a lightly greased container, cover, and let rise in a warm spot for 1 hour or until doubled in size.
Place the sheet pan of baking soda in the oven for 1 hour while the dough rises.
When the dough has doubled in size punch it down to deflate it slightly, cover it, and let it rest for another 10 minutes. Meanwhile, remove the baking soda from the oven and set aside to cool.
Divide the dough into 8 equal sized pieces, using a kitchen scale if you want to be precise.
To shape the dough into rounds, gently flatten a piece of dough on a clean, unfloured surface. If you're combining multiple pieces of dough, stack the smaller pieces on top of the biggest piece so you can tuck them inside. Fold the top of the dough down over the middle, rotate the dough 45° and repeat. Keep going all the way around the dough, folding the top edge down over the middle until you have a smooth surface against your counter and the "seam" side facing up.Flip the dough over so the smooth side faces up. Cup your hand gently around it with your pinkie against the counter. Slide your hand toward your body to push the dough ball closer to you without rolling it. This will increase the surface tension on top of the dough and shape it into an oval. Rotate the dough 90° and repeat the sliding motion to turn the oval into a circle.Repeat with the rest of the dough. Cover with a damp paper towel and let rest 10 minutes.
Dust your hands with flour and use your thumb to poke a hole up through the bottom seam of each dough round and out the other side. Rotate the dough through your hands as you squeeze it to stretch the bagels out. You want the dough to stretch, not tear. The holes should be fairly large — twice the width of the bagel sides.
Arrange the shaped bagels on a silicone mat or parchment lined sheet pan and cover with a damp paper towel. Refrigerate for 30 minutes.
Preheat oven to 420°F with an oven rack in the middle position.
Just before the bagels finish proofing in the fridge, fill a large, high-sided skillet about halfway with water (about 8 cups, depending on the size of your skillet). Bring it to a low boil on the stove. In a separate bowl, combine 5 cups cool water with all of the baked baking soda. Whisk well to make sure all the baking soda has dissolved.
Working in batches of 2 or 4, boil the bagels 1 minute per side. Use a wire spider to transfer the boiled bagels directly into the baking soda bath, 10-15 seconds per side. Remove the bagels from the baking soda bath and plae them back onto the sheet pan. Sprinkle them generously with pretzel salt while they're still wet on top.
Bake for 20 minutes until deeply browned and shiny.Let bagels rest 5-10 minutes on the sheet pan, then transfer to a wire cooling rack to finish cooling completely.
To quickly soften your 2 tablespoon of butter, slice it into half-inch cubes and smoosh it with the tines of a fork against a clean cutting board until it's smooth and soft.
Measure your baking soda by weight before you bake it. It loses 30% of its weight when you bake it so if you weigh it after it bakes you'll have way too much baking soda in your water bath.
Make sure the baking soda is fully dissolved in the water bath before you dip the boiled bagels in it. If there are lots of clumps or the baking soda hasn't dissolved, your bagels might end up tasting soapy.
If you don't have a mixer with a dough hook, follow the same instructions, just start mixing the dough in a bowl with a wooden spoon and then knead it on a lightly floured surface by hand for 8-10 minutes once the dough comes together.