Preheat the oven to 350°F (175°C).

In a bowl, cream together the softened butter and granulated sugar until light and fluffy.

Beat in the egg and vanilla extract until well combined.

In another bowl, mix together the flour, baking powder, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Divide the dough into portions and add gel food coloring to each portion, swirling to create a marbled effect.

Roll the dough into egg shapes and place on a lined baking sheet.

Bake for 8-10 minutes or until lightly golden.

Allow the cookies to cool completely before icing.

To make the icing, beat together the powdered sugar, egg white (or meringue powder), and lemon juice until smooth and spreadable.

Decorate the cooled cookies with icing.