In the bowl of a stand mixer, add butter and brown sugar. Mix on medium-high for several minutes until fluffy and light.
2 cups butter, 1 cup light brown sugar
Add 3½ cups of the flour and mix on medium-low speed until combined.
4 and ½ cups all-purpose flour
Sprinkle ½ cup of flour on your clean countertop. Knead dough for 5 minutes, adding as much of the remaining ½ cup flour as needed to make a soft dough. The dough should be soft but thick, not sticky. (You may not end up using all the flour, depending on how your dough feels.) Divide the dough in half, wrap tightly, and chill for at least 30 minutes or until firm enough to roll.
When you're ready to bake the cookies, preheat the oven to 325℉. Line your baking sheets with a silicone mat. Place enough Nutella in a sandwich-size Ziploc bag to fill it about halfway. Carefully snip off the corner to make a piping bag.
¾ cup Nutella
Work with your two batches. Keep one refrigerated while you're working with the first one. Roll the chilled dough into about a 1-inch ball. Place the ball on your cookie sheet. Using the back of a spoon, form a well in the center of your cookies. Place the cookies 2 inches apart on the baking sheet. Bake for 8 to 10 minutes - They won't get very brown. You can depress the centers again if needed. Fill the well with the Nutella you add to the zip-top bag.
You can add sprinkles to the top if desired. Store in an airtight container. You can refrigerate the dough for up to a week or freeze it up to a month.