In the large bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with a hand mixer, beat together the butter and vanilla sugar for about three minutes, until fluffy. Add the egg and vanilla extract and beat until smooth.
In a medium bowl, sift together the flour, baking powder, and salt.
Turn the power to low and mix in the flour mixture just until the dough comes together.
Scoop tablespoonfuls of dough and gently roll them into rounds in your hand. Lay them on the cookie sheet, about two inches apart, and press down slightly.
Bake for about 12 minutes until the cookies just begin to turn golden around the edges. Remove cookies from the oven and let sit for about two minutes, then move them to a wire rack and let cool completely.
Make the glaze. In a small bowl, whisk together the powdered sugar, milk, vanilla extract, and vanilla bean seeds until smooth.
Dip the top of each completely-cooled cookie into the glaze, set back on wire rack and let the glaze set (about 15 minutes). I like to place a paper towel or two under the wire rack for easier clean-up.