Preheat the oven to 350°F and line muffin tins with 14 liners.

In a medium mixing bowl, whisk together flour, baking powder, and salt. Set aside.

In the bowl of a stand mixer with paddle attachment or a large mixing bowl with a hand mixer, cream the butter and sugar together until light and fluffy, 2-3 minutes, scraping down the sides. Add the vanilla and mix until just combined.

Add the flour mixture in three parts, alternating with the milk and strawberry purée, scraping down the sides as needed, mixing until just combined.

In a medium bowl, whip the egg whites to soft peaks with a whisk or hand mixer. Carefully fold the egg whites into the batter, until just combined.

Fold in the chopped strawberries.

Evenly divide the cake batter among the 14 cupcake liners.

Bake for 23-26 minutes (for gluten-free, bake 25-27 minutes), rotating the pans after 15 minutes, until the center springs back when gently poked and a toothpick comes out clean or with just a few dry crumbs. Let cupcakes cool completely on a wire rack before stuffing and frosting.

Meanwhile, prepare the strawberry compote for the buttercream. Reduce the strawberry purée. In a small saucepan over medium-low heat, cook the strawberry puree, stirring constantly, until thick and bubbly and reduced to 3-4 tablespoons, about 10-15 minutes. Transfer to a bowl and place in the fridge until completely cool.

Once cupcakes are cool, use a small knife to cut an angled 1-inch circle out about 1-inch deep out of the top of each cupcake to create a hole for filling. Spoon about 1 teaspoon of Nutella into each pocket.

Prepare the rest of the strawberry buttercream. In the bowl of a stand mixer fitted with the paddle attachment or a large bowl with a hand mixer, cream the butter over medium speed. With the mixer on low, slowly add the powdered sugar to the butter, in 3 batches, beating until incorporated. Add the vanilla extract and cooled strawberry compote and mix until smooth. Add the milk, starting with just 1 teaspoon and adding additional milk as needed, until frosting is smooth and spreadable.

Pipe the frosting onto each cupcake, covering the Nutella pocket.

Garnish with a halved strawberry.