Put the milk, yeast, egg and one cup of the flour in the bowl of a mixer. Mix just until blended with a rubber spatula. Sprinke the remaining flour over the sponge. Set the sponge aside to rest uncovered for 30 to 40 minutes. After this resting, the flour covering the top will crack, indicating that the sponge is progressing as it should. Add the sugar, salt, eggs, and one cup of the flour to the sponge. Using the bread hook begin to mix the sponge on low for one minute or two, just until the ingredients come together. While continuing to mix add an additional 1/2 cup of flour. Once the flour is incorporated bring the mixer speed to medium for 15 minutes stopping only to scrape down the sides. During this time, the dough should form, coming together and slapping the sides of the mixing bowl as the dough hook spins. To mix in the butter, it must be of a similar consistency as the dough. To accomplish this you can put it in plastic wrap and beat it with a rolling pin, or using a dough scraper blend it back and forth until smooth and soft. Now slowly add the butter to the dough a couple tablespoons at a time. The dough will look like it is separating, don't worry. That's normal. When all the butter has been added, increase the speed of the mixer to medium-high for one minute. Reduce the speed to medium and beat the dough for five minutes. Clean the sides of the bowl as you go. If the dough doesn't look like it is coming together, add one to two tablespoons of flour. When the dough is ready, it will be somewhat cool, soft and slightly sticky.