Chocolate, Cinnamon, & Hazelnut Thumbprints

Name: Chocolate, Cinnamon, & Hazelnut Thumbprints
Cuisine: European, German
Ingredients: 1/2 cup toasted hazelnuts; 2 cups flour; 1/2 cup unsweetened cocoa powder; 1/2 tsp baking soda; 1/4 tsp salt; 1 1/4 tsp ground cinnamon; 1 cup unsalted butter, at room temperature ((2 sticks)); 1 cup sugar; 1 egg; 2 tsp vanilla extract; 2 tsp chocolate extract ((see note)); 1/4 cup sugar; 1/2 tsp ground cinnamon; 1/4 cup unsalted butter, at room temperature ((1/2 stick)); 4 oz bittersweet chocolate ((I used 60% cacao)); 2 Tbsp toasted hazelnuts, chopped; 2 oz bittersweet chocolate, melted ((I used 60% cacao))