In a small bowl, whisk together the flour, cocoa powder, baking soda and kosher salt. Set aside.
2 cups all purpose flour, ½ cup cocoa powder, 1 teaspoon baking soda, ½ teaspoon kosher salt
In a stand mixer fitted with the paddle attachment, or in a large bowl with a handheld mixer, combine the butter and both sugars. Cream on medium-high speed for 2 minutes until light and fluffy.
½ cup unsalted butter, ½ cup light brown sugar, ½ cup granulated sugar
Stop the mixer and scrape down the sides and bottom of the bowl with a rubber spatula. Add the egg and vanilla extract. Beat on medium-high speed for 1 minute.
1 large egg, 2 teaspoons vanilla extract
Stop the mixer and add the dry ingredients. Mix on low speed, then add the tablespoon of milk. Stop the mixer once everything is combined, you don't want to over-mix it!
1 tablespoon whole milk
Scoop out 1 tablespoon of cookie dough and roll between your hands to form a ball. Roll the cookie dough ball in the chopped hazelnuts, then place on a parchment lined baking sheet or plate. Repeat with all of the dough, you should get 35 cookies. Use your thumb, the end of a wooden cooking spoon or spatula or the back of a measuring spoon to create the indent.
¾ cup roasted hazelnuts
Chill the cookies in the fridge for 1 hour.
After 1 hour, preheat the oven to 350°Fahrenheit/175°Celsius and line 2 baking sheets with parchment paper. Place the cookies on the sheet, leaving a few inches of space between each cookie. Bake 1 sheet at a time for 11-13 minutes in the middle of the oven.
Take the cookies out of the oven, they should look set with some cracks.
If the indent doesn’t look as big as before, gently press down with the bottom of a teaspoon or end of a spatula. Use a small cup, glass or biscuit cutter to gently scoot the cookie into a perfect circular shape again.
Allow the cookies to cool on the baking sheet for 10 minutes before removing to a wire rack to cool completely.
Measure out the nutella into a heat-safe bowl.
½ cup nutella
In a saucepan, or in a bowl in the microwave, heat the heavy cream over medium-low heat until it begins to simmer but not boil.
¼ cup heavy cream
Pour the hot cream over the nutella and stir until smooth.
Spoon into the thumbprint cookies or place in a bag and pipe it out.
Notes
My recipes have been developed and tested in metric grams. I highly recommend using a weighing scale and measuring out your ingredients the metric way. Butter: I use European-style butter in my recipes. This will make your finished bake much creamier and richer.Salt: If you don’t have kosher salt, halve the amount of salt called for in the recipe. 1 teaspoon kosher salt = ½ teaspoon fine salt. Please note I am using Diamond Kosher salt and not coarse kosher salt!Nutella: You don't have to use branded Nutella, you can use any chocolate hazelnut spread. Hazelnuts: Try to chop up the hazelnuts finely so they stick to the cookie dough balls. Large pieces of chopped hazelnuts won't stick as well! Storage: store the baked cookies in an airtight container for up to 5 days. You can freeze the baked cookies for up to 2 months, just let them thaw to room temperature.