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1
Preheat oven to 350 degrees F (175 degrees C). Line a 9x13-inch baking pan with aluminum foil; grease with cooking spray.
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2
Combine white cake mix, milk, melted butter, and vanilla pudding mix in a large bowl; beat with an electric mixer until well blended and thick. Divide dough into 2 pieces.
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3
Place 1 piece of dough between 2 large sheets of waxed paper; roll into a 9x13-inch rectangle. Peel off waxed paper and press the dough into the bottom of the baking pan.
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4
Bake in the preheated oven until dough is pale golden, about 10 minutes. Sprinkle half of the sliced almonds, white chocolate chips, white baking chocolate, and hazelnuts on top; press into the soft dough.
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5
Place chocolate hazelnut spread in a small microwave-safe bowl; heat on medium-high, stirring often, until melted, about 1 minute. Spread over dough.
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6
Place the second piece of dough between 2 large sheets of waxed paper; roll into a 9x13-inch rectangle. Peel off waxed paper and lay the dough over the chocolate hazelnut spread. Sprinkle remaining sliced almonds, white chocolate chips, white baking chocolate, and hazelnuts on top; press into dough with a spatula.
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7
Bake in the preheated oven until dough is golden brown, about 15 minutes. Let cool completely, about 1 hour. Refrigerate until firm, 3 to 4 hours. Cut into bars.
