I followed a "recipe" from Kirbie's Cravings, but I figured out some tips out along the way that make it easier. From the top, cut the strawberries in half about three-quarters of the way down. Use a pastry bag with a flat tip (like the basket weave or rose petal tip). This makes it easier to get the Nutella in the strawberry since you're not trying to fit a huge round tip into the middle. I squeezed out about 1/2-1 teaspoon of filling into each berry and moved the bag from left to right so that it would squeeze out through the sides where I cut.
If you don't have pastry bags, you could put some nutella in a sandwich bag, cut off the tip and squirt it in. I think it'll be messier because you won't have the stability/control of the tip, but I'm sure it can be done. You also could add more Nutella, but I loved having just the taste of it inside the berry.
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Pint of strawberries, rinsed
1/2 cup Nutella
Pat the strawberries dry and cut off the leaves/stem. This will give the strawberry a flat surface to sit on. From the top, cut the berries in half but only cut about three-quarters of the length down.
Fill a pastry bag fitted with a flat tip with the Nutella. Gently open the strawberry from the top, place your pastry bag inside it (about half-way down), and fill the strawberry with 1/2-1 teaspoon of Nutella while moving the bag from left to right so the filling comes out of the cuts. You can add more filling if you'd like. Repeat with the remaining strawberries.
Source: Kirbie's Cravings