Preheat oven to 350 degrees. Lightly spray 8 x 8″ pan with oil. Line pan with parchment paper to the top edges of the pan. With a stand or handheld mixer, cream butter, sugar and salt until fluffy, approx. 3-4 min, scraping down the sides and bottom. Add yolks, one at a time, then vanilla and mix until incorporated. Add flour all at once and mix at low speed until combined; do not overwork. Press dough into prepared pan and level the surface. Prick with a fork in several places and bake 15-18 min, until edges are golden brown. Allow to cool completely before proceeding with next steps.
For peppermint layer, place powdered sugar in a bowl, add boiling water and extract then mix with a rubber spatula until smooth. Add crushed candy and mix well. Working quickly, spread over cooled crust and level. Refrigerate 1 hr.
Chop chocolate and place into microwaveable bowl. Heat 1 min on 60% power. Mix with a rubber spatula then heat again 30 sec, mix then heat another 30 sec. Add oil and blend well. If there are lumps, heat again 30 sec, being careful not to scorch. When smooth, pour over peppermint and quickly spread into an even layer to cover. Allow chocolate to harden.
Lift parchment paper to remove from pan. Using a large, sharp knife, slice into bars of desired size and serve.
* Callebaut chocolate is available in your Market Bakery retail area.