1. Soak raisins in the rum in a small bowl at room temperature for at least 4 hours or overnight, stirring occasionally.

2. Place half-and-half, salt, and 1/4 cup sugar in a large heavy saucepan. Scrape the seeds from the vanilla beans into the mixture, add the beans, and bring to a boil over medium-high heat. Remove from the heat, and let the vanilla beans steep for at least 1 hour or until the mixture cools to room temperature.

3. Beat egg yolks and the remaining 1/2 cup sugar using an electric mixer until thick and pale-lemon in color. Mix in flour.

4. Reheat the half-and-half mixture, and discard the vanilla beans. Gradually beat the mixture into the yolks. Stir in the rum-soaked raisins.

5. Return the mixture to the saucepan. Whisk over medium-high heat until the mixture comes to a boil and becomes very lumpy. Reduce heat to medium low, and whisk mixture until smooth, 2 to 3 minutes.

6. Preheat oven to 400 degrees. Fill the prebaked tart with the custard, and smooth the top with a spatula. Brush with egg. Transfer to oven, and bake until the custard is well browned, 20 to 25 minutes. Cool in the tart pan on a rack for 3 to 4 hours.

7. To serve, remove the outer ring of the tart pan. Cut 6 to 8 wedges, and transfer them to dessert plates. Reheat the rum sauce, stirring over medium-high heat. Ladle over tart slices. Serve immediately.