Prepare Meringue Cups: Preheat the oven to 225°F (108°C). Using a large glass, trace six large circles (roughly 3 inches in diameter) on a large sheet of parchment paper. Place parchment paper on a large baking sheet–pencil side facing down.
Place the egg whites in the bowl (clean and free of any grease) of stand mixer, fitted with a whisk attachment. Whisk over low speed for 1 to 2 minutes, or until egg whites are foamy. Increase speed to medium and continue to whisk for an additional 1 to 2 minutes. Slowly start sprinkling in the granulated sugar, while continuing to whisk at medium speed. Increase speed to high and whisk until meringue reaches a stiff peak.
Whisk in the vanilla bean paste. Place the meringue in a pastry bag fitted with a star tip. Starting in the center of the circle, pipe a layer (roughly ½-inch thick) of meringue. Trace the edges of the circle four times to form the sides of the meringue cups. Repeat until all meringue cups are formed.
Immediately (and carefully) place the meringue in the oven and bake for 1 hour and 30 minutes. Once baked, turn off the oven and allow meringues to dry in the warm oven for an additional 20 to 30 minutes. Meringue should be crispy, hard, and not sticky. If the meringue is wet or sticky (which can depend on humidity levels), allow them to bake for an additional 10 to 15 minutes. Remove the meringue cups from the parchment paper and set aside.
Roast Rhubarb: Preheat the oven to 400°F (204°C). Line a large baking sheet with parchment paper or foil. Toss the rhubarb with the sugar, orange juice, and zest. Allow mixture to sit for 10 to 15 minutes to macerate. Roast the rhubarb for 15 to 20 minutes, or until it is cooked through, but can still retain its shape. Remove and allow to cool completely.
Vanilla Bean Whipped Cream: Combine the heavy cream, powdered sugar, Grand Marnier, and vanilla bean paste. Whip to soft peaks.
Assembly: Right before serving, fill the meringue cups with whipped cream and top with roasted rhubarb pieces. Garnish with orange zest. Serve immediately.
Tips for Success:
Meringue shells can be prepared up to a day or two in advance and kept in an airtight container at room temperature (preferably with a silica gel pack to keep moisture out).