Add the milk, 1 tablespoon of sugar, and the yeast to the bowl of a stand mixer and give it a quick stir. Allow it to stand for 5 minutes until the yeast activates and becomes a foamy layer on top.
1 cup whole milk, ½ cup granulated sugar, 1 tablespoon active dry yeast
Add the remaining sugar, egg, mashed banana, butter, and vanilla and mix with the dough hook on low speed.
1 large egg, ½ cup mashed banana, ½ cup unsalted butter, 1 teaspoon vanilla paste or extract
Add in 5 cups of the flour (600g) and salt, and mix on low to combine and moisten. The dough will look very loose and wet at this point.
5 ½ cups all-purpose flour, ½ teaspoon kosher salt
Scrape any excess flour down from the sides, then mix the dough on medium high speed for five minutes. The dough will be very smooth at this point.
Add in the remaining ½ cup of flour. Mix on low until the flour is absorbed, then mix on medium-high for another minute until all the dough pulls away from the sides of the bowl.
Note: The dough should be very elastic and may be tacky, but should not be sticky. If it's sticky, add another ¼ cup of flour and mix for an additional minute. Repeat as necessary until the dough comes together.
Grease a large mixing bowl and transfer the dough to the bowl using your hands or a spatula. Cover with plastic wrap or a dish towel and allow it to rise in a warm place until doubled in size. About one hour. Alternatively, cover with plastic wrap and allow it to rest and rise in the fridge overnight.