In a medium bowl, whisk together the flour, cocoa powder, salt, and cinnamon; set aside.

In a large mixing bowl, beat butter and powdered sugar on medium speed until fluffy, about 4 minutes. Beat in the egg and vanilla until fully incorporated.

With the mixer on low speed, gradually add the flour mixture until just combined.

Roll the dough into 1 1/4-inch balls, then roll each ball in sanding sugar. Place 2 inches apart on parchment-lined baking sheets.

Use your thumb to gently press an indentation into the center of each dough ball.

Chill the shaped cookies for 30 minutes while preheating the oven to 350°F (175°C).

Bake for 8–10 minutes, rotating sheets halfway through. Let cool on baking sheets for 15 minutes.

Press the indentations again if they have puffed during baking.

In a small bowl, heat cream until just steaming. Add chopped chocolate and let stand 2 minutes, then stir until smooth.

Let the ganache cool for 5 minutes, then spoon about 1 teaspoon into each cookie’s center.

Chill cookies for 10 minutes to set the filling before serving.