The concept behind the sponge method is to create a starter or pre-ferment. According to Britannica, in the mass production of enriched bread, the sponge is made of about ½ to ¾ of flour, the entirety of the yeast, yeast foods like salt or sugar, and plenty of water to make the sponge stiff. It’s then mixed to allow the gluten time to form and stretch. This process is easily transferable to home baking. The sponge is continually kneaded during the mixing process until it reaches a stage where light can easily pass through it. This is called the window-pane test. Only once the sponge has reached this point can the remainder of the ingredients be added. Any remaining flour is mixed in, along with the butter, via MasterClass.
The sponge allows the dough a head start in the fermentation process. As with traditional sourdough, getting the bread to ferment and stretching the gluten enables a complexity of flavors to develop. This method can be used when making any type of butter bread, such as brioche or challah. It’s advisable to chill your dough during its initial rise as the butter fats weaken the already stretched gluten molecules, making it very difficult to knead and shape at room temperature.