Preheat oven to 350 degrees F (175 degrees C). Line 2 baking sheets with parchment paper.

Beat butter and sugar with an electric mixer until fluffy, about 4 minutes. Add egg and vanilla, beating until incorporated.

Whisk together flour, cocoa, salt, and cinnamon in a medium bowl. Add to butter mixture and beat until combined, about 2 minutes.

Roll dough into 1 1/4-inch balls, roll in sanding sugar, and place on baking sheets 2 inches apart. Press the center of each ball with your thumb.

Bake cookies in the oven, rotating sheets halfway through for 8 to 10 minutes.

Let cookies cool for about 15 minutes on baking sheets. Press the indentations again if puffed.

Heat cream in a microwave until steaming, about 45 seconds. Add chocolate and stir until melted, then let cool for 5 minutes.