Take flour, baking powder and salt in a bowl and mix well. Set aside.
Take butter, powdered sugar and vanilla in a bowl and mix till fluffy.
Add in egg and mix till combined.
Add in the flour and mix gently.
Form it into a smooth dough.
Divide it into three portions. Add green and red food colours in two portions and mix well.
Now take the white portion and roll between cling wrap. Do the same to the other two colours.
Place the red rolled over the white and then the green.
Now roll the whole thing together to form a large swiss roll like.
Now cover with plastic wrap and chill in the fridge for 1 hour.
Now preheat oven to 180 degree C.
Remove the rolled cookies, cut into slices about a inch thick.
Place on a parchment lined pan.
Bake for 12 to 15 mins.
Remove and cool it down completely.
Store in a air tight container.