How to Make It
Prepare the Cookies: Beat butter in a stand mixer fitted with a paddle attachment on medium speed until creamy, about 30 minutes. Gradually add sugar, beating until light and fluffy, about 1 minute. Add egg yolks 1 at a time, beating until incorporated after each addition. Beat in vanilla.
Stir together flour, cocoa, and salt in a bowl. Add flour mixture to butter mixture, beating on low speed until just combined, about 30 seconds.
Preheat oven to 300°F. Shape dough into 1-inch balls. Arrange balls 2 inches apart on 2 baking sheets lined with parchment paper. Press thumb into center of each ball to create an indentation.
Working in 2 batches, bake in preheated oven until set, 15 to 18 minutes. Remove from oven, and gently re-press thumb into indentions. Cool on baking sheets 5 minutes. Transfer to wire racks; cool completely, about 15 minutes.
Meanwhile, prepare the Praline Filling: Stir together brown sugar and half-and-half in a small saucepan. Cook over medium-high, stirring constantly, until mixture comes to a boil. Continue boiling, stirring constantly, 4 minutes. Remove from heat; stir in butter and salt. Whisk in powdered sugar until smooth, about 1 minute. Use immediately (filling hardens quickly).
Working with 1 Cookie at a time, fill each indentation with Praline Filling, and immediately sprinkle with some of the pecans. Let stand until set, about 20 minutes. Store in an airtight container between layers of wax paper or parchment paper up to 1 week.