In a medium bowl, stir together sugar, stevia, cornstarch, nutmeg, and salt. Add yolks and stir until smooth.
In a medium saucepan, bring milk to a simmer. Slowly whisk about half the milk into the yolk mixture, then whisk yolk mixture into remaining milk in saucepan. Place back on heat and cook, stirring constantly, just until thickened. Strain through a fine sieve into a medium bowl and stir in butter and rum.
Press a piece of plastic wrap directly on surface of pastry cream and refrigerate until well chilled, at least 2 hours.
Preheat oven to 400 degrees F.
In a small bowl, stir together sugar and stevia. Lay out phyllo dough in a stack on a flat surface and cover with a dry kitchen towel, then a damp kitchen towel.
Place 1 sheet of phyllo in front of you, brush lightly with butter, and sprinkle lightly with sugar-stevia mixture. Place another phyllo sheet on top and brush with butter and sprinkle with sugar-stevia mixture. Repeat this layering until there are 8 layers of phyllo, then brush top layer with butter and sprinkle with sugar-stevia mixture.
Cut phyllo lengthwise in half to make 2 long rectangles. Cut rectangles crosswise every 2 inches. Transfer as many of the rectangles as possible to a large baking sheet, leaving a little space between them. Nest another baking sheet of the same size on top so the phyllo rectangles are pressed between the baking sheets.
Bake until browned and crisp, 12 to 16 minutes. Transfer to a rack to cool completely and repeat with any remaining phyllo rectangles, baking and cooling them completely.
In a medium bowl, beat cream until stiffly whipped. Stir pastry cream just until smooth, then fold in whipped cream.
To assemble, lay 1/3 of the phyllo rectangles on a countertop in front of you. Spread a layer of pastry cream over each rectangle, then top with another phyllo rectangle. Spread another layer of pastry cream over phyllo, then top with a final phyllo rectangle. There should be 3 phyllo layers and 2 pastry cream layers.
Dust each Napoleon with a little grated nutmeg and serve immediately.
2 tablespoons Sugar In The Raw®
2 tablespoons Stevia In The Raw®
1 tablespoon cornstarch
¼ teaspoon ground nutmeg
¼ teaspoon sea salt
3 egg yolks
¾ cup whole milk
1 tablespoon cold unsalted butter, cut into small pieces