Preheat oven to 350˚F (180˚C) and line a large round pan with parchment or lightly grease.
In the bowl of a stand mixer fitted with a dough blade or in a large bowl, beat together the butter and sugar until light and fluffy. Beat in the vanilla sugar if using followed by the eggs one at a time.
Mix in the flour and salt just until combined. Be careful not to overmix.
Press the dough into a large, even circle about 1/2 inch (1 cm) thick on the prepared round pan. Using your thumb and forefinger, pinch around the outside of circle to create a scalloped edge.
In a small bowl, beat together the egg yolk and water. Brush the pastry with the egg wash, then use a fork to create diagonal lines across the top. If desired, sprinkle with the sliced almonds.
Bake in the preheated oven until golden brown and firm, 25-30 minutes. Allow to cool completely to room temperature on a wire rack.
To serve, either use your fist to punch the center and break into pieces or slice. Store in an airtight container at room temperature for up to 1 week.