Step 1 In a saucepan over medium heat, warm the milk to 105 to 110 degrees. Pour it into a large bowl, add the yeast and stir to dissolve. When the yeast gets slightly foamy (after about 10 minutes), stir in one-half cup of flour. Cover with plastic wrap; set aside for an hour in a warm place.
Step 2 Heat the oven to 350 degrees. Place four eggs, the yeast mixture, 2 1/4 cups of the flour, and the salt and sugar in the bowl of a 5-quart stand mixer. With the paddle attachment, beat the mixture on medium speed until well mixed. Continuing on medium speed, slowly add in the butter and truffle butter.
Step 3When completely mixed, the dough should be fairly wet but pull away cleanly from the sides of the bowl. Add flour as necessary, up to 1 cup. When the mixture starts to pull together, switch to a dough hook and knead for a few minutes.
Step 4Remove the dough from the mixer and shape into six flattened balls about half the size of the buns that you want. Place the balls on a greased and floured cookie sheet, leaving space for them to rise. Cover with a damp towel or plastic wrap sprayed with cooking spray. Place the buns in a warm place, and allow them to rise until doubled in size, about 1 hour.
Step 5Brush the buns with an egg wash -- one egg beaten with 1 tablespoon of water. Bake for about 25 minutes, or until the tops are a deep golden brown and a skewer inserted into the middle comes out clean. Cool on a wire rack before cutting.