Spread pecan halves on a parchment lined baking sheet in a single layer and bake for 10 to 12 minutes, or until roasty and fragrant. Remove from the oven and cool. Transfer the pecans to a cutting board and give them a rough chop. Allow chopped pecans to cool completely.
In a medium bowl, whisk together the flour, cocoa powder, and salt. In the bowl of a stand mixer fitted with the paddle attachment, beat together the butter, half of the sugar (1/4 cup), and vanilla until pale and fluffy, 2 to 3 minutes. Scrape down the sides of the bowl with a spatula and add the flour, cocoa powder and salt mixture. Mix on low until all of the dry ingredients are incorporated. Add the pecans and the sprinkles and mix until just combined.
Line a work surface with a large piece of parchment paper. Scrape the dough out onto the parchment paper and gather it together with your hands. Roll into a 6 1/2-inch-long log that is about 2 inches wide. Tightly wrap the dough in parchment paper, twisting the ends so it’s airtight. Refrigerate until the dough is firm, for 2 hours or overnight.
Set an oven rack in the middle position. Preheat the oven to 325 degrees F. Line a baking sheet with parchment paper and set aside.
Spread the remaining granulated sugar on a plate and roll the log in the sugar to coat completely.
Slice the log into eighteen 1/4-inch discs and place them on the baking sheet evenly spaced apart. Bake in the center of the oven until the cookies are no longer shiny and slightly puffed; begin checking for doneness at 16 minutes. Let cool on the pans.
Using a 1/2-ounce scoop or a spoon, dollop about 1 tablespoon of the filling on the bottom of 12 of the cooled cookies. Sandwich with another half and smush lightly. Enjoy immediately or store in an airtight container on the counter until you eat them all.
Cream together butter and caramel until smooth. Add powdered sugar and salt and whip until combined and light and fluffy, 2 to 3 minutes.