Directions

1. Preheat oven to 350 degrees. In the bowl of an electric mixer fitted with paddle attachment, cream butter and sugar on medium until fluffy. Add egg yolk, vanilla, and salt; beat until combined, scraping down sides of bowl as needed. Add flour, and beat on low speed until just combined. Stir in chocolate. (Dough can be frozen, wrapped in plastic, up to 1 month; thaw before using.)
2. Form dough into 1-inch balls; place on parchment-lined baking sheets about 2 inches apart. Using thumb or rounded end of a wooden spoon or dowel, make a deep indentation in the center of each cookie. Fill each with about 1 teaspoon creme de marrons or Nutella. Place in freezer or refrigerator until firm, about 15 minutes.
3. Bake until edges are golden and cookies are firm, about 12 minutes. Transfer cookies to a wire rack to cool completely. Store in an airtight container, at room temperature, up to 3 days.