In a bowl of an electric mixer fitted with a paddle attachment, add 4 cups of flour, butter, brown sugar, and salt. Mix on low for a few seconds to combine. Stop the mixer and add the yeast mixture, eggs yolks, milk, and orange zest. Mix on medium to combine. Scrape the dough from the mixer and dump onto a lightly floured surface. Knead until the dough, sprinkling the remaining 1/4 cup of flour as needed until dough is smooth and elastic. The dough should feel slightly tacky to the touch, but not sticky, about 5-7 minutes. Alternatively, you can mix the dough in your stand mixer using a dough hook over medium speed until the dough is smooth and elastic.