Preheat the oven to 350°F (175°C). In a mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.

Add the egg, vanilla extract, baking powder, and salt, and mix well. Gradually add the flour and blend until a smooth dough forms.

If the dough is soft, chill it in the refrigerator for about 15 minutes. Roll out the dough and use cookie cutters to shape the cookies.

Place the cookies on a baking sheet and bake for 12-15 minutes or until golden brown. Let them cool completely.

In a clean bowl, combine powdered sugar, egg white, and lemon juice. Beat until smooth and glossy.

Decorate the cooled cookies with the icing as desired, letting it set before serving.