Melt the butter in a medium saucepan over low heat.
Add the water and sugar. Bring the temperature to 105℉. (I highly recommend using an instant read digital thermometer).
Stir in the yeast. Let rest 10 minutes. The yeast should bloom and foamy bubbles will appear on the surface of the water mixture.
To the bowl of a stand mixer fitted with the dough hook attachment, add the flour, salt, sourdough starter, and water mixture.
Mix the dough on low for 7 minutes. The dough should be fully combined and stretchy when the mixing is complete.
Cover the bowl with a damp towel or plastic wrap. Let rest for 40 minutes.