Cream the butter and sugar until light and fluffy. Add the egg, egg yolk, vanilla, and blue raspberry oil and mix until smooth.
Add the blue gel coloring gradually, using a four-to-one ratio of sky blue to royal blue, until the dough reaches a bright electric blue.
Whisk the flour, baking powder, baking soda, and salt together, then add to the dough and mix just until combined. Fold in the rough-chopped white chocolate. Portion the dough using a 1 oz scoop.
Roll each portion lightly in blue shimmer or coarse sanding sugar, then gently press into a small puck shape. Freeze until solid.
Bake straight from frozen at 350°F for 11 to 13 minutes. The edges should be set and the centers just soft. Let the cookies rest on the baking sheet for 2 to 3 minutes before moving.
