Mix flour, whey powder, baking powder and baking soda in a large mixing bowl. In another bowl, mix butter until creamy, then beat in erythritol, stevia, egg, egg white and vanilla. Combine the flour mix into the butter mix, bit by bit until a crumbly dough forms. (Mix well to avoid dough separating later) Press into 2 balls, roll the first until it's about 1/4" thick. Use cookie cutters to cut out the shapes and place them on cookie sheet. Keep re-rolling and cutting out the dough until it's used up. Decorate cookies with sprinkles and crushed candy cane. Bake at 350 degrees fahrenheit for 5-7 minutes. Repeat with second dough ball.