Makes 2 cups
1. Pour water to depth of 1 inch into bottom of a double boiler over medium-high heat; bring to a boil. Reduce heat to medium-low, and simmer. Whisk eggs and egg yolks in top of double boiler off of heat; whisk in sugar and next 3 ingredients. Scrape out vanilla bean seeds into egg mixture, and whisk until blended. Place top of double boiler over simmering water. Cook, whisking constantly, 10 minutes or until mixture thickens.
2. Remove top of double boiler from simmering water, and add butter and salt, whisking until butter is melted. Place plastic wrap directly on warm curd (to prevent a film from forming), and chill 8 hours.