Mix together the flour, ground almonds and icing sugar in a bowl.
Add the cold, cubed butter and vanilla extract and rub them in using your fingertips until well combined and no lumps of butter remain.
Bring the mixture together into a soft dough and knead gently just a couple of times until it is smooth. Wrap and refrigerate for a couple of hours until the dough is firm.
Divide the dough into 20 pieces, I use a scale for accuracy (about 16g each) and roll them into balls.
Roll each ball into a small log shape, tapering the ends slightly. Shape the log into a crescent. Lightly flour the surface if necessary.
Place the crescents spaced apart on a couple of baking parchment lined baking sheets and pop them in the freezer while you preheat the oven to 180°C/160°C fan/350°F/gas mark 4.
Bake the cookies for 12-15 minutes, until golden around the edges and on the underside but still pale on top.
Mix the icing sugar and vanilla sugar (if using) in a wide, shallow bowl.
Allow the cookies to cool on the baking sheets for 5 minutes, then gently roll them in the icing sugar while they are still warm. Place on a wire rack to cool completely.