Preheat oven to 350˚F (180˚C). Line two baking sheets with parchment paper or lightly grease.
In a large bowl, combine the flour and baking powder.
Mix in the softened butter with your hands until the texture becomes sandy.
Mix in the sugar, vanilla sugar, and whole egg just until a soft dough comes together.
On a large work surface, roll the dough into a rope about 20 inches (51 centimeters) long.
Cut the dough into slices about 3/4 inch (2 centimeters) thick.
Roll each slice of dough into a smooth ball and place on the prepared baking sheets 2 inches (5 centimeters) apart.
Use a fork to lightly press down on the top of each round. If the edges crack, gently press them back together.
Brush each cookie with the egg white and sprinkle some chopped almonds and sparkling sugar or pearl sugar over the top.
Bake in the center of the preheated oven just until set and lightly golden on the bottom, 12-14 minutes. If baking both sheets at once, rotate halfway through.
Transfer the cookies to a wire rack to cool to room temperature. Once cooled, store in an airtight container at room temperature for up to a week.