In the bowl of stand mixer fitted with the paddle attachment combine flour, baking powder, sugar, vanilla extract, egg, milk, and butter (cut the butter into cubes) at medium-high speed for about 2 minutes until a slightly soft but not sticky dough forms. Don't mix too long or the butter will get too warm and it will be hard to work with the dough.
Wrap the dough in plastic wrap and let rest in the fridge for about 10 minutes while making the filling.
Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper or a non-stick baking mat.
To make the filling: In the bowl of your stand mixer fitted with the paddle attachment combine the egg white, ground hazelnuts, sugar, almond extract, and water. Beat at medium speed until combined.
Take the short pastry out of the fridge and roll the dough into a 10x20-inch (25x50cm) rectangle on a lightly floured surface. The dough should be about 0.2 inches (1/2 cm) thick.
Spread the hazelnut filling onto the dough and distribute it evenly.
Starting along a long side, tightly roll up the dough.
To make a braided roll: With a sharp knife, cut the roll in half lengthwise. Carefully turn the halves so the cut sides are facing up, and then loosely twist the halves around each other, keeping cut sides up.
Carefully transfer the twisted rope to the baking sheet and shape into a wreath. Pinch the ends together to seal.
Brush the roll with beaten egg yolk milk (and make a few cuts into the roll when you didn't braid it).
Bake it until lightly golden brown, about 40-45 minutes.
If your short pastry is too crumbly you can add one additional egg.