Fit a stand mixer with a dough hook, or gather a large bowl if mixing dough by hand.
Add the sweet levain, tangzhong, milk, egg, honey, sugar, and vanilla to the bowl. Give it a stir- it's okay if it's lumpy.
275g (1 cup + 2 Tbspns) whole milk, 200g (3/4 cup) sweet levain, 1 large egg, 25g (1 Tbspn) raw honey, 60g (1/4 cup + 1 Tbspn) white granulated sugar, 1 tsp vanilla extract, All of the tangzhong paste
Add 500g all-purpose flour and salt. Mix on low or knead with your hands until a rough dough forms.
500-550 g (4-4 1/4 cups) all purpose flour, 1 tsp (6g) sea salt
Add butter 1 Tablespoon at a time working butter into dough slowly with mixer or hands until all the butter is added,
57g (1/4 cup) unsalted butter, softened
Continue mixing on low or knead with hands 8-10 minutes and dough is smooth and elastic. If dough is very wet, add 25g of flour to make it less sticky. Work flour into dough and add up to 50g (do not exceed) at this step. Dough should be tacky not sticky.
Spay a clean bowl or clear container with oil, and transfer dough to bowl. Cover with plastic wrap.
If using stand mixer, perform 1 set of stretch and folds after 30 minutes.If mixing by hand, perform 3 sets of stretch and folds at 30 minute intervals at the beginning of bulk fermentation.