Chocolate Thumbprint Cookies

Stages of cooking

1. Preheat oven to 350 degrees F (175 degrees C). Position racks in the upper and lower thirds of oven. Line 2 baking sheets with parchment paper.
2. Beat butter and sugar at medium speed with an electric mixer until fluffy, about 4 minutes. Add egg and vanilla, beating until incorporated.
3. Whisk together flour, cocoa, salt, and cinnamon in a medium bowl until combined. With mixer on low speed, gradually add flour mixture to butter mixture, beating until well combined, about 2 minutes.
4. Roll dough into 1 1/4-inch balls, then roll balls in sanding sugar. Place on the prepared baking sheets, spacing 2 inches apart. Press the center of each ball with your thumb (about 1/2-inch deep and 1/2-inch wide).
5. Bake in preheated oven in upper and lower thirds, 2 baking sheets at a time, rotating top to bottom and bottom to top halfway through, until cookies are set and almost firm, 8 to 10 minutes.
6. Let cool on baking sheets, about 15 minutes. When cool, press the center of each ball with your thumb again—the indentations will have puffed in the oven.
7. Heat cream in a small microwavable bowl on HIGH until just steaming, about 45 seconds. Remove from the heat, and add chopped chocolate. Let the mixture sit, undisturbed, for 2 minutes. Stir until melted and smooth. Let cool until slightly thickened, about 5 minutes. Place about 1 teaspoon filling in the center of each cookie. Slide the cookies into the refrigerator for about 10 minutes to set the filling. The cookies will keep for about 5 days in an airtight container at room temperature.