First, you need to have all your ingredients at room temperature, including the eggs and the Nutella.
Preheat the oven to 325F.
Sift together the flour, baking powder and salt. Set aside.
Place the sugar and butter in a large mixing bowl. With a wire whisk or a mixer, beat the butter and sugar together. Just keep beating until the mixture is light and creamy.
Add the eggs, one at a time, beating after each addition.
Add the flour mixture and mix. The mixture may appear lumpy at first but keep right on mixing until you have a smooth batter. Note that the batter will be heavy and hardly pourable. Stir in the vanilla.
Meanwhile, line the muffin pan with paper cups. Use an ice cream scoop (very practical) to fill the pans until about 3/4 full.
Top with a heaping teaspoonful of Nutella. Take a toothpick, dip into the unbaked cupcake until the tip hits the bottom of the pan.
Using a scooping motion, swirl the Nutella with the batter so that some of the batter comes up on top of the Nutella.
Bake in a preheated 325F oven for exactly 20 minutes.
Serve warm or cooled.