Preheat the oven to 350 degrees F. Coat a 12 by 17 by 1 inch rimmed baking sheet with cooking spray, line with parchment and lightly spray the parchment, too.
In the bowl of a stand mixer, beat together butter and sugar until light and fluffy, about 2 minutes. Beat in vanilla. Beat in eggs one at a time, scraping down the sides of the bowl.
Turn the mixer to low, and add in the flour. To avoid over-beating, stop mixing when a few streaks of flour remain in the batter and stir by hand until no flour streaks remain.
Scrape the batter into prepared pan and smooth it into an even layer. Microwave the Nutella for about 20 seconds, or until its lightly melted. Drizzle the melted Nutella onto the batter and swirl it around using a toothpick. Evenly place the sliced strawberries on top of the batter.
Bake for 20-30 minutes, or until a toothpick inserted in the center comes out clean. Transfer the pan to a wire rack and cool completely. Just before serving, cut shortbread into 24 squares and dust with powdered sugar.
Store leftovers in an airtight container between layers of waxed paper or parchment. Squares will keep for up to 3 days.