Take eggs, milk, and butter out of the fridge and let it come to room temperature. Cut butter into cubes and set aside.
Combine flour, sugar, yeast and salt in a large mixing bowl.
Add eggs and milk and mix the dough on low speed with a mixing paddle just until it gets lumpy. Switch to the dough hook and knead the brioche dough on medium speed for about 2 minutes.
Scratch the dough down the bowl and off the hook, then mix it another 2 minutes. At this point, your brioche dough sticks to the dough hook. Scratch down the dough again.
Mix butter piece by piece into the brioche dough on medium/low speed. Then turn speed up to medium and mix the brioche dough for about 4 minutes. Scratch down the dough again and mix it another 4 minutes. Now, your brioche dough is silky and shiny.
Take the dough out of the bowl with a dough scraper and place it onto a very lightly floured work surface and knead the dough per hand a few times. Form a ball out of it and place it into a lightly greased bowl. Cover your brioche dough and let it rise in a warm place for about 1 hour until doubled in size.
Knead the dough again with your hands and let it rest for another hour.
Meanwhile, you can make your sweet poppy seed filling. Heat milk, sugar and cinnamon in a small saucepan. Just bring it to a boil, remove the saucepan from the stove top and stir in poppy seeds. Let it cool. Stir occasionally.
Preheat oven to 190°C (375°F).
Knead your brioche dough a few times by hand and divide it into 8 equal pieces. Take one piece, roll it into a little ball and flatten it into a disc with your hands.
Grab a spoonful poppy seed filling, place it in the middle of your flatten dough, flap the edges to the middle and completely close it with your fingers. Repeat this with every piece.
Place all brioche buns onto a parchment paper layered baking sheet and bake it for about 17-19 minutes until they are light brown. Don't overbake them; otherwise, your brioche buns will dry out.
Let it cool on a wire rack.
Combine powder sugar and water in a small bowl and brush your brioche buns with it.
You can store your poppy seed filled brioche buns in an airtight container for up to 2 days.