Heat oven to 350 degrees F. Line baking sheets with silpats (French nonstick baking mats), or parchment paper.
In the bowl of an electric mixer fitted with the paddle attachment, beat butter and sugar on medium until light and fluffy. Add egg yolk, vanilla, and salt; beat until combined. Add flour and mix on low speed until just combined. Remove bowl from mixer and, using a wooden spoon, stir in chocolate.
Using a 1 1/4-inch ice cream scoop or a tablespoon, drop dough onto prepared baking sheets and roll into 1 1/4-inch balls, spacing about 2 inches apart. Make a thumbprint in the center of each cookie. Fill each thumbprint with 1/2 teaspoon creme de marrons.
Bake until edges are golden and cookies are firm, 12 to 15 minutes. Transfer cookies to a rack until completely cool. Store in an airtight container up to 3 days.
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