The first step in making the brioche feuilletee, or flaky brioche, is to get the brioche dough prepared.Heat milk in a microwave safe bowl until it reaches 110 degrees F. Make sure to use a thermometer! Then gently stir in yeast and a teaspoon of sugar and let it sit and get foamy for 15 minutes.Meanwhile, set up your stand mixer fitted with the dough hook. In the bowl of the mixer, combine the flour, salt, and sugar. Then make a well in the middle. Add yeast mixture, eggs, and vanilla extract. Use a wooden spoon to incorporate the ingredients from the inner well, bringing the flour mixture into the beaten egg mixture.Then use the dough hook to mix the dough on medium speed for 10 minutes.Add the softened butter a tablespoon at a time, mixing well for a minute or two after each addition.Then turn the mixer to medium high and mix for 5 more minutes.Then butter a large bowl and use a spatula to turn the dough into that bowl. Cover with plastic wrap and allow the dough to double in size, which takes about 60-90 minutes.*If you are making this overnight flaky brioche then leave the dough tightly wrapped in the fridge overnight for about 12 hours.BUTTER BLOCKFor the butter block use dry butter that is at least 84% fat. This entire recipe uses European butter.Arrange the butter onto parchment paper and set another piece of parchment on top. Gently tap the butter block until it’s an 8 x 8 square. Chill for 30 minutes.LAMINATING THE BRIOCHEBrioche Feuilletee is all about layers within flaky layers. I made a few plain buns below to show you the lamination and how they rise from within if you bake them separately on a baking sheet rather than in a loaf pan.When the dough has doubled in size, lightly flour a surface and turn the dough onto that surface. Flatten the brioche dough and gently flatten it into a square. Then roll it out to a rectangle that is about 16″ x 24″. Place the butter block in the middle of the rectangle. Then bring the bottom third up and over the butter. Bring the top third down over that. Essentially, fold it like a letter. Then use the rolling pin to gently push on the dough to seal in the butter block, pinching the seams as needed to conceal butter. Roll it out to seal in the butter. Turn it 90 degrees and repeat the letter folding process. Cover tightly with plastic wrap. Chill 20-30 minutes.Bring the dough out of the fridge. Roll it into a rectangle once again. Then repeat the folding and chilling process once more.CHOCOLATE HAZELNUT FILLED BRIOCHE*The following recipe gives you the option of filling the flaky brioche with chocolate hazelnut spread, like Nutella, or having it plain. For the plain version, brush on melted butter in place of the chocolate hazelnut spread.For the last roll, roll it very thin so it is about 1/16 inch thickness. After you have rolled it out for the last time, either brush it with melted butter or spread chocolate hazelnut spread over the dough, if using.Then use a pizza or pastry cutter to cut the dough through the middle so you have two even pieces. Trim the sides as necessary so you have straight edges and even pieces. Roll up each side from the short side until you have two big layered rolls. Chill 20 minutes. Then slice each roll in half with a sharp knife so you have 4 equal portions.Butter and line 2 9 x 5 loaf pans. Set parchment with overhang inside for easy lifting, if desired. Then place two flaky brioche buns into each pan. Cover and let rise for another hour.Preheat oven to 375 degrees.Bake the brioche loaves for 25-30 minutes, until crisped and golden. Dust with powdered sugar after they have cooled.