Combine the beer, sugar, and kosher salt in the bowl of a stand mixer. Sprinkle the yeast on top and let sit for 5 minutes or until the yeast begins to foam (the beer will be foamy as well).
Add the flour and butter and mix using the dough hook attachment until well-combined. Change to medium speed and continue to knead until the dough is smooth, 4-5 minutes.
Oil a large bowl and transfer the dough in a ball to the bowl. Cover with plastic wrap and let sit for 1 hour, or until doubled in size.
Preheat the oven to 450°F. Line 2 half-sheet baking trays with parchment paper and set aside. Bring 10 cups of water and ⅔ cup baking soda to a boil in a large pot.
In the meantime, turn the dough out onto a smooth, dry work surface (no need to flour or oil, you'll want some traction). Divide into 8 equal pieces. Roll each piece into a 24-inch rope. Make a U-shape with the rope, then cross the ends of the rope over each other like a heart. Twist the ends together and push down at the bottom of the U. Place on the parchment-lined baking pan.
Place the pretzels in the boiling water, one by one, for 30 seconds each. Remove them from the water using a slotted spatula and return to the baking sheet. Brush each pretzel with the egg wash and sprinkle with flaky sea salt.
Bake until golden brown, approximately 13-15 minutes. If your oven heats unevenly (like mine!) you may want to rotate and switch the pans halfway through so they all get even color. Transfer to a cooling rack to cool. Serve with a mustard or your favorite dipping sauce.