1. Preheat oven to 350 degrees.
2. Cream butter, sugars and chocolate together for 3-4 minutes, until light and fluffy.
3. Add the egg and vanilla and beat until well combined.
4. Add the flour, baking soda and cornstarch and mix until smooth.
5. Scoop balls of about 1 tbsp of dough out onto a cookie sheet lined with a silicone baking mat, or parchment paper.
6. Bake for 8-9 minutes.
7. Remove from oven and allow to cool for about 3 minutes, then remove to cooling rack to finish cooling.
8. Once cooled, make the frosting. In a large saucepan, combine the egg yolks, milk and vanilla and whisk until well combined.
9. Add sugar and butter and cook on medium heat for 12-15 minutes, or until thickened (almost pudding-like) and golden brown, stirring constantly.
10. Remove from heat and add coconut and pecans. Mix well.
11. Allow to cool completely.
12. Using three cookies per stack, layer the cookies into stacks with the coconut pecan frosting. Each stack should have three layers of frosting. If the frosting is too thin, add more coconut until thickened. Refrigerate cookies until ready to serve.