Preheat oven to 375 degrees F. Sift flour, cocoa powder and salt into a bowl and set aside.
In the bowl of an electric mixer, beat butter and sugars until fluffy (scraping the bowl when needed), for about 3-4 minutes. Add in egg yolk and vanilla, and beat for another 1-2 minutes until combined. With the mixer on low speed, stir in the flour mixture and gradually increase the speed for the dough to come together, again scraping down the sides to combine. One batter has come together, fold in chocolate chips.
Roll into balls that are about 1 inch in diameter and set on a baking sheet 2 inches apart. Bake for 5 minutes, then remove from oven and gently press your thumb into the center of each cookie, creating a dent. Place the cookies back in the oven and bake for 5-6 more minutes. Remove and let cool completely.
One cool, use a spoon or butter knife and add 1-2 teaspoons of frosting into the cookies. Top with a hefty sprinkle of graham cracker crumbs. Store in an air-tight container. My grandma used to freeze her thumbprints, so I’m pretty sure these can be frozen if placed in a single layer and wrapped tightly.
Notes: I used a bit of extra milk (2-3 tablespoons) to thin out my usual marshmallow frosting. If you are making these ahead of time and plan to freeze, I don’t suggest that as the frosting is a bit runnier. If you are making them a day or two in advance, thinning the frosting should be fine.