In a medium bowl, whisk together flour, cornstarch, baking powder, and salt.

Divide the flour mixture into two halves.

In a small bowl, whisk together egg, milk, and vanilla extract.

In a separate bowl, cream sugar and softened butter until light and smooth.

Beat the egg mixture into the creamed butter until just combined.

Turn the dough out onto a lightly floured board and gently knead into a soft dough.

Wrap the dough in plastic wrap and refrigerate for up to 3 days or freeze for up to 6 months.

Preheat oven to 350°F (180°C).

Work with about 1 tablespoon of dough at a time.

Roll each portion of dough into a ball.

Roll each ball into a 3-inch length with tapered ends.

Turn the ends down to form a crescent shape.

Repeat with the remaining dough.

Place the dough crescents about 1 1/2 inches apart on a prepared cookie sheet.

Bake for about 16 minutes, or until golden.

While the cookies are still warm, dip them in sifted icing sugar.

Arrange the cookies on a wire rack to cool to room temperature.

Dip the cooled cookies again in icing sugar.