In a small saucepan, warm the milk and butter, until just simmering (you don't want it to scorch.)
In the bowl of a stand mixer fitted with a paddle attachment. Combine 2½ cups of the flour, yeast, sugar and salt. Stir until combined.
Add the egg yolks and milk/butter mixture. Gradually add the rest of the flour, ½ cup at a time until a soft dough forms. Switch to the dough hook and knead for 5-7 minutes or until a soft, smooth dough is formed that springs back lightly when pressed.
Place the dough in a lightly oiled bowl and cover with a clean towel. Let it proof in a warm place for about 40 minutes or until doubled in size.
Divide the dough into 4 equal sections. Cover the ones you aren't using, and work with one at a time.
Shape the dough into a circle and roll out to a 10-inch circle. Spread a layer of nutella onto the dough, avoiding the very outer edge.
Repeat 2 more times with the remaining dough/nutella, rolling each section out, placing it on top, and spreading the nutella on. Top with the final dough round.
Use a plate to create one perfect circle, lightly tracing with a sharp knife and then trimming the dough away.
Divide the dough into 16 sections, leaving a 3 inch circle in the middle. Use a sharp knife to cut the sections.
Twist each section and then press the edges together two at a time. (See pictures below to make this more clear.)
Brush pastry with egg whites. Bake at 350F for about 20 minutes, or until pastry is golden. Serve warm or room temperature.
Notes
Recipe adapted from Handmania, (and the video I saw on Facebook!)
Recipe by The Baker Chick at https://www.thebakerchick.com/braided-nutella-pastry/