1. Place milk and vanilla bean seeds in a medium saucepan and bring to a boil.

2. Meanwhile, in a medium bowl, sift together flour, sugar, and cornstarch. Add eggs, and whisk mixture until pale and stiff. Continue whisking while slowly adding half of the hot milk mixture. Transfer egg mixture to saucepan with the remaining milk mixture. Place over medium-high heat, whisking constantly, and bring to a boil. Cook for 2 minutes.

3. In a small bowl, whisk together chestnut paste and rum, if using. Remove egg mixture from heat, and whisk in chestnut paste mixture. Pour mixture into a wide shallow dish, and cover the surface directly with plastic wrap to prevent a skin from forming. Refrigerate until cool. Strain through a fine mesh sieve before using.