Begin by preheating your oven to 350°F or 175°C. Lightly grease or line a 9×13 inch baking pan with parchment paper. In a large bowl, cream together the butter and sugar until light and fluffy. Add the egg and vanilla extract, and mix until smooth. In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet mixture until a soft dough forms. Press the dough evenly into the prepared baking pan. Bake for 20 to 25 minutes, or until the edges are lightly golden and the center is set. Allow the bars to cool completely before decorating. Once cooled, divide your frosting into small bowls and color each with different pastel food dyes. Spread the frosting across the bars and decorate with mini eggs and colorful sprinkles. Cut into squares and serve.