**Prepare Nutella**: Spoon Nutella into small discs on a baking sheet and freeze for at least 1 hour.
**Make Cookie Dough**: In a large bowl, cream together butter, brown sugar, and granulated sugar until light and fluffy. Add eggs and vanilla, mixing until well incorporated.
**Combine Dry Ingredients**: In a separate bowl, whisk together flour, baking soda, and salt. Gradually add the dry mixture to the wet ingredients, mixing until just combined.
**Shape Dough**: Use a cookie scoop to form dough balls. Make a small indent in the center of each ball and place a frozen Nutella disc inside. Seal the dough around the Nutella.
**Chill Dough**: Refrigerate the dough for 1.5 hours to prevent spreading.
**Bake**: Preheat oven to 350°F (175°C). Place dough balls on a lined baking sheet, about 2 inches apart. Bake for 9-10 minutes until the edges are golden but the center remains soft.
**Cool & Serve**: Let cookies cool on the baking sheet for 10 minutes. Sprinkle with flaky sea salt while still warm. Enjoy!