1. Heat oven to 350˚, line 8x8 pan with parchment paper hanging over both edges.
2. Combine flour, powdered sugar, salt, nutmeg, and butter in bowl of food processor. Pulse until it is sandy.
3. Scatter dough into prepared pan, press into even layer.
4. Bake crust for about 30 minutes, until golden brown and firm. Remove from oven and set aside while you prepare the filing.
5. Reduce oven temperature to 300˚.
6. Beat egg yolks and sugar together with electric mixer until pale and thick.
7. Gradually beat in cream, rum, salt, and ¼ teaspoon ground nutmeg.
8. Pour filling over crust. Bake for 30-40 minutes, until custard is set and knife inserted in center comes out clean. Remove from oven and cool in fridge completely before decorating.
9. Cut into squares or diamonds. Drizzle with caramel sauce, then sprinkle white chocolate shavings on top.