Preheat oven to 350 degrees and line a cookie sheet with parchment paper
Start by adding butter and 1 1/4 cups sugar in a large mixing bowl and mix until light and creamy
Add in the egg and egg yolk to the butter and sugar mixture and mix to combine
Mix in the vanilla and salt
In a separate bowl, mix together the flour and baking powder and then slowly incorporate into your sugar batter .. this takes time, it will seem very dry at first but will come together nicely
Separate your batter into 3 separate but equal parts and into individual bowls. Add blue food coloring to one bowl and red food coloring to another bowl and stir to combine or even use your hands to mix the coloring into your dough *suggest wearing disposable gloves if you are using your hands.
Once your color(s) are incorporated into your dough, pinch off a small amount of the red dough, the blue dough and the non-colored dough so that combined they make a ball about the size of a golf ball. Lightly roll the dough together to make a ball then break the ball in half and rip off some of the dough and place it in a different spot and roll the ball back together. You are looking to mix up the color pattern, that is what will create the 'swirl effect'.
Add 1/2 cup of sugar to a separate small bowl.. this is the sugar you will roll your cookie in before baking.
Once you are happy with your color placement, roll your dough ball though the extra sugar and place them on your cookie sheet - do no flatten!
Bake for 10 to 12 minutes and remove.. let sit on the cookie sheet for 2-3 minutes before moving the cookies to a wire rack
Let cool and enjoy!!