Mix the flour, baking powder and a pinch of salt together. Add the butter and rub in with your fingers until the mixture resembles coarse crumbs. Mix in all the icing sugar and the vanilla sugar, then add the whole egg and mix until just combined, but be careful not to overmix.
Transfer the dough to a food bag and put in the fridge to chill for about 1 hr.
Heat the oven to 180C/160C fan/gas 4. Roll the chilled dough into about 40 even balls (around 15g each). Arrange the dough balls over two baking sheets lined with baking parchment, leaving space between each for spreading. Using the back of a fork, press the balls down gently in the middle, so they spread to a diameter of about 3cm (they will spread more as they bake). If you prefer flat cookies, use the bottom of a glass.
Brush the cookies with the beaten egg white and sprinkle over the pearl sugar or almonds. Bake for 10 mins, or until just cooked through (the cookies should not be fully browned, just lightly golden at the edges). Remove from the oven and transfer to wire racks to cool completely.