Pour the flour into the bowl of a stand mixer fitted with the paddle attachment.  Add the salt and yeast on opposite sides of the bowl, stirring in each one with your finger.  Make sure they don't touch each other directly, as the salt can kill or stunt the yeast.

Dump in the softened butter and half of the water, then turn the mixer on low speed.  Gradually trickle in the remaining water and keep mixing until a smooth, sticky dough forms.  You may not need all the water, or you might need a little extra, depending upon the brand of flour.

Switch the attachment to a dough hook and knead on medium-low speed for 3-5 minutes, until the dough passes the windowpane test.  Test it by breaking off a lump of dough and stretching it between the thumb and forefinger of each hand to create a windowpane.  If the dough stretches to be translucent without breaking, it's kneaded enough; if not, knead for a minute longer and check again.

Shape the dough into a ball, place in a buttered bowl, and cover tightly with plastic wrap.  Let the dough prove at room temperature until at least doubled in size, about 1 ½ to 2 hours.

Turn the dough onto a floured surface and punch it down all over to knock out pockets of gas.

Shape the dough into a rectangle about 9 inches wide, then fold the two ends into the middle.  Roll up the loaf tightly from one of the long sides to create the bloomer.  Pinch the seam to seal, then place it seam-side down on a large cookie sheet.

Place the loaf inside a large, clean garbage bag and inflate the bag.  Tuck the ends underneath the tray to seal.  Let the loaf prove at room temperature until almost doubled in size, about 1 hour.

Just before baking the loaf, remove the garbage bag and cut a few diagonal slashes on the top of the loaf with a sharp serrated knife or baker's lame.

Place the loaf in the oven, then immediately pour some tap water into the hot metal pan underneath the loaf and quickly shut the oven door.  This will create steam, which will give the loaf a hearty crust.  Bake at 425 F for 30 minutes.

Allow the loaf to cool on a wire rack for at least 45 minutes before slicing and serving.  The cooling time allows the bread to finish cooking all the way through.  Cutting it too soon will give you a gummy interior.