Instructions

1. In a large bowl, stir together the melted butter, brown sugar and granulated sugar with a wooden spoon. Then add the eggs and vanilla extract, stirring until well combined.
2. Warm the Nutella in your microwave for 20-30 seconds and then add it to your bowl, stirring to combine it with the other ingredients. Add the flour, baking powder and salt and stir until the flour is fully incorporated and no more white streaks remain. Cover the bowl with plastic wrap and place it in your refrigerator to chill the dough for at least 2 hours or up to overnight.
3. Pre-heat your oven to 350 degrees. 
4. Form the chilled cookie dough into balls, using about 1 1/2 to 2 tablespoons of dough for each ball, and place the balls about 1 1/2-inches apart on parchment paper lined baking sheets. Transfer the baking sheets to your pre-heated oven and bake the cookies for 8 minutes. Then rotate the pans and bake the cookies for an additional 3 minutes. The centers may still look slightly underdone, but the edges should be firm.
5. Remove the baking sheets from your oven and let the cookies rest on their baking sheets for 5 minutes. Then transfer them to a wire rack to continue cooling. 
6. Once all the cookies are baked and cooled, make the ganache. Place the chopped chocolate and Nutella in a medium bowl. Set aside. In a small saucepan, warm the heavy cream over medium heat just until small bubbles start to form around the edges. Do not let the cream come to a full boil. Pour the warmed cream over the chopped chocolate and Nutella and let it sit for 1-2 minutes. Then whisk until the chocolate is fully melted and the mixture is glossy and completely smooth. 
7. Let the ganache cool for 10-15 minutes. Then spoon about a tablespoon of the ganache over the top of each cookie. Decorate as desired with sprinkles, candies, nuts, etc.
8. These cookies can be stored in an airtight container at room temperature for up to 1 week.