Whisk 1/2 cup milk, 1/3 cup flour, and 1/2 cup water in a small saucepan until smooth. Set over medium heat and cook, stirring contsnatly, until you have a very stiff paste (something that looks like mashed potatoes - reference above images). This process takes me about 3 minutes. Scrape the mixture into the bowl of a stand mixer fitted with the dough hook attachment.
Add the sour cream, sugar, kosher salt, 2 eggs, 4 Tbsp butter, and the remaining 5 cups (650g) bread flour, and the yeast to the bowl with the milk mixture. Beat on low speed until a shaggy dough forms, about 1 minute.
Increase the speed to medium and mix until a smooth and supple dough forms. This process takes me about 8 to 10 minutes. Tip: if your dough seems a little bit too sticky, add more flour, 1 Tbsp at a time, until the dough is no longer too tacky.
Scrape the dough out onto an unfloured work surface, and form into a ball. Place the dough back into the same bowl and cover with plastic wrap. Let it sit in a warm place (I put mine in a cold oven with the oven light on) until it's roughly doubled in size, about 1 to 1 1/2 hours.
While the dough is rising, prepare your 13 x 9 inch cake pan. Use an offset spatula to smear 2 Tbsp butter across the bottom and on the sides of the pan.