Lay 1 tortilla half on a clean work surface with the rounded side closest to you. Brush the edges of the tortilla with egg. Place 1 tablespoon of chocolate hazelnut spread in the center of the tortilla and place a piece of fruit or marshmallow on top. Fold one side of the tortilla half way over the filling so that the corner is in the 6-o'clock position, press to seal. Fold the other side over the filling so that the two edges overlap slightly and the corner is also in the 6-o'clock position, press to seal. The quesadilla will look like a triangle-shaped hand pie. Repeat with the remaining tortilla halves and fillings.
Melt half the butter in a large nonstick skillet over medium heat and cook half of the quesadillas until deep golden brown, about 2 minutes per side. Transfer to a paper towel-lined plate. Wipe the skillet clean and repeat with remaining butter and quesadillas. Dust with confectioners' sugar before serving.
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