Lay 1 phyllo sheet on a work surface. Brush the phyllo with melted butter. Top with another phyllo sheet. Repeat with more butter and 2 more phyllo sheets. Cut the stacked phyllo sheets into 6 (5-inch) squares. Line each muffin cup with 1 stacked phyllo square, allowing the edges to ruffle and extend above the cup rims. Repeat with the remaining 4 phyllo sheets and melted butter. Bake until the phyllo cups are golden brown, about 9 minutes. Transfer the muffin pan to a cooling rack and cool completely.
Meanwhile, stir the white chocolate and 1/4 cup cream in a heavy small saucepan over low heat until the chocolate is melted and smooth. Pour into a large bowl and cool to barely lukewarm. Stir in the hazelnuts. Using an electric mixer, beat the remaining 1 1/4 cups cream in another large bowl to medium-firm peaks. Fold the cream into the white chocolate mixture in 2 batches. Cover and refrigerate until cold, about 1 hour.
Spoon 2 teaspoons of the chocolate hazelnut spread into each phyllo cup, if using. Spoon the white chocolate mousse into the cooled phyllo cups, mounding slightly. Arrange the mousse-filled phyllo cups on plates. Sprinkle the bittersweet chocolate over the mousse, or garnish with edible flowers, and serve.
Do-Ahead Tip: The unfilled phyllo cups can be made 2 days ahead. Store them in an airtight container at room temperature. The white chocolate mousse can be made up to 8 hours ahead. Keep it refrigerated. The filled phyllo cups can be assembled up to 1 hour ahead. Cover and refrigerate them.