In a medium mixing bowl, use a hand mixer to beat the egg yolks until they become pale and creamy. With the mixer on low, gradually add the sweetener and beat until fully combined.
Set aside.
Pour the almond milk and 1 teaspoon (5 ml / 0.2 fl oz) of nutmeg into a medium saucepan.
Bring the mixture just to a boil over high heat, stirring occasionally to prevent scorching.
Remove from the heat immediately.
Very slowly, temper the eggs by gradually pouring about half of the hot milk mixture into the egg yolk mixture while whisking constantly. This prevents the eggs from scrambling.
Once combined, pour the warmed egg mixture back into the saucepan with the remaining milk.
Cook over moderate heat, stirring constantly with a spatula, until the mixture thickens enough to coat the back of a spoon or a thermometer reads 160°F (71°C).
Do not let it boil.
Remove the custard from the heat and stir in the rum extract.
Pour it into a clean bowl and place it in an ice bath to cool quickly.
Once at room temperature, transfer to the refrigerator to chill completely, for at least 1 hour.
Just before serving, beat the egg whites in a separate clean bowl with a hand mixer until stiff peaks form. Gently fold the beaten egg whites into the chilled custard until just combined.
Pour into four glasses, garnish with a sprinkle of extra nutmeg, and serve immediately.
