Preheat your oven to 350°F or 175°C and line a baking sheet with parchment or a silicone mat. In a large bowl, cream together the softened butter and granulated sugar until light and fluffy. Mix in the egg and vanilla extract until well blended. In a separate bowl, whisk the flour, gelatin, baking powder, and salt. Gradually add the dry mixture to the wet mixture until a soft dough forms. Scoop out tablespoon-sized portions of dough, roll into balls, and place on your prepared baking sheet 2 inches apart. Gently flatten each cookie and optionally roll in colored sugar or top with sprinkles. Bake for 8 to 10 minutes or until edges are just set and centers remain soft. Let cool for 5 minutes on the tray, then transfer to a wire rack. Once cooled, decorate with royal icing if desired.