Preheat the oven to 350F and prepare a sizable baking sheet by covering it with parchment paper.
Combine the dry ingredients in a medium bowl and set them aside.
In a larger mixing bowl, beat the softened butter and sugars until they form a paste-like texture.
Incorporate the egg and vanilla until the mixture is smooth.
Gently fold in the dry ingredients, chocolate chips, and chopped Terry's orange chocolate using a spatula until fully integrated—take care not to over-blend.
Scoop out 3 tbsp size balls of dough onto your cookie sheet.
Bake for 10-12 minutes.
Once out of the oven, add extra Terry's chocolate orange slices on top.
Allow to cool on a wire rack and serve!