Warm the milk to 105 - 110 degrees ferenheight. Add the yeast to the milk and allow to sit and bloom for 10 min.
Combine milk mixture, to flour and mix until no dry bits remain.
Let sit for 45 - 1 hr
Dough
Add sponge to the mixing bowl of your stand mixer.
Add eggs, sugar, 2nd addition of flour, salt, using a hook attachment of your stand mixer, mix on medium speed until fully incorporated.
Increase speed to medium-high (speed 7-8) and continue mixing until dough begins to pull away from the sides of the bowl, approximately 10 - 13 min (depending on the power of your mixer). The dough will start to look shinny and will be very elastic.
In the last 1-2 min of the previous step mixing begin adding butter a few pieces at a time untill fully incorporated. Continue mixing for another 13 - 15 minutes. The dough will begin to pull away from the sides and you will hear a slapping sound.
Continue mixing until dough passes the window pane test.
Once the dough has passed the window pane test and has formed a nice taught ball. The dough will also have inched its way up the dough hook attachment. Remove dough from the bowl and begin forming into a tight ball. Place the dough into a lightly oiled bowl and cover.
Allow dough to sit in a warm area for 2 hr until doubled in size.
Shapping
Once the dough is fully proofed, de-gas (punch down) and remove from bowl on to a lightly floured surface.
Divide dough into 4 equal size pieces.
Take each piece, stretch into a rectangle and fold the top half to the center and the bottom half over the top half. Rotate a ¼ turn and pat down the dough. Repeat the process and then cup hands around sides to make a tight log.
Repeat the step above with the remaining sections of dough until you have four taut logs.
Place each section into your well oiled loaf pan and cover with plastic wrap. Allow to proof until doubled in size. (risen to the top of the loaf pan)
Brush loaf with egg wash. Bake in a 375° preheated oven for 20 - 25 min.
Notes
Window pane test: take a small amount of dough and stretch until you can see through it without it tearing.