• 1/4 c. orange marmalade, divided
• 6 oz. bittersweet chocolate
Preheat oven to 375 degrees and line baking sheets with parchment paper.
In a bowl of an electric mixer on medium-high speed, beat butter until creamy, about 1 minute. Add sugar and beat until light and fluffy, about 2 minutes, scraping sides of bowl occasionally. Add egg yolk and orange zest, and beat until thoroughly combined. Reduce speed to low, add flour and mix until just incorporated.
On a lightly floured work surface, using a lightly floured rolling pin, roll dough to 1/4-inch thickness. Using a 1 1/2-inch round cookie cutter, cut dough into rounds, and place cookies 1 inch apart on prepared baking sheets. Repeat with remaining dough, gathering up scraps, re-rolling and cutting until all dough is used. Using your thumb, make a slight indentation in center of cookie and fill with 1/4 teaspoon orange marmalade. Bake until edges are lightly browned, about 12 minutes. Remove from oven and cool 2 minutes before transferring cookies to a wire rack to cool completely.