Heat the oven to 350°F (175°C). Line an 8 x 8-inch square pan with parchment paper, leaving overhang on two sides to create a sling.

Beat the softened butter and granulated sugar until creamy. Beat in the egg and vanilla extract until well combined.

Whisk the flour, graham-cracker crumbs, baking powder, and salt in a separate bowl. Add the dry mixture to the butter mixture and mix just until a soft dough forms.

Press about half of the dough evenly into the prepared pan. Spread the Nutella over the dough while leaving a small border around the edges.

Scatter the miniature marshmallows evenly over the Nutella. Flatten pieces of the remaining dough between your fingers and place them over the marshmallows to make a mostly covered top layer.

Bake for 30–35 minutes, beginning to check at 30 minutes, until the top is lightly golden and the center appears set. Do not overbake because the bars should remain soft and gooey.

Cool the bars in the pan for at least 30 minutes before lifting them out with the parchment sling and cutting into 16 squares. For cleaner slices, briefly chill the cooled bars before cutting.