In a mixing bowl, whisk together flour, cocoa powder and salt, set aside. In the bowl of an electric stand mixer fitted with the paddle attachment, whip butter, granulated sugar and brown sugar on medium-high spread until slightly pale and fluffy, about 2 minutes. Mix in egg yolk (set egg white aside in a bowl in refrigerator) and vanilla extract.
Blend in buttermilk. With mixer set on low speed, slowly add in dry ingredients and mix just until combined.
Shape dough into a ball and drop onto a sheet of plastic wrap, flatten dough into a 6-inch disk then wrap with plastic wrap and chill 45 minutes, until slightly firm (or up to 1 day. If chilling longer than a few hours, I'd recommend letting the dough rest a room temperature a bit so the dough isn't too firm and cracking when making indentations).
Preheat oven to 350 degrees. Remove egg white from refrigerator and whisk vigorously until frothy. Place finely chopped pecans in a bowl.
Shape dough into 1-inch balls (18 grams each) then working with one at a time, drop into egg white and roll to evenly coat the lift and allow excess egg white to run off and immediately place in bowl with chopped pecans and roll to evenly coat (if necessary, you can gently press into the nuts to help them stick).
Transfer to parchment paper lined baking sheets and make a deep indentation with thumb in the center of each dough ball.
Bake in preheated oven until set, about 10 - 12 minutes. Remove from oven then using the bottom of a rounded teaspoon, gently press down on existing indentation (to create more space for caramel). Cool on baking sheet several minutes then transfer to a wire rack to cool completely.
Place caramels and heavy cream in a microwave safe bowl. Heat mixture in microwave on HIGH power in 30 second intervals, stirring after each interval until melted and smooth. Spoon caramel into indentation in cookies and sprinkle tops with a small pinch of sea salt.
Recipe adapted slightly from Cook's Illustrated
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