In the bowl of a stand mixer, add the water and starter and mix with a fork.
Add the sugar, butter, and flour. On top, add the salt.
Mix on low for 2-3 minutes then on medium for 5-6. Should not be sticky and should be able to see through with the window-pane test. If not, mix for another 2-3 minutes.
On a lightly floured surface, roll out dough into a rectangle. Roll up and tuck ends under and place in an oiled- or parchment-lined 9x5 pan.
Cover with a shower-cap or plastic wrap and let rise until doubled and the crown is a bit above the edge of the pan.
Heat oven to 400℉ or 375℉ convection.
Right before baking, slice the top and brush with butter, pouring any excess into the sliced center.
Bake uncovered for 30-35 minutes. Thermometer should register 190℉ in center of loaf.
Remove from pan, turn off oven, and return loaf to oven rack with door cracked if you want a crustier crust.