For the panna cotta

1. Prep gelatin. Pour 1/2 cup milk into a medium bowl and sprinkle with gelatin. Let sit until gelatin softens, about 10 minutes.
2. Infuse cream. While gelatin softens, combine cream, sugar, lemon peel, and remaining 1/2 cup milk in a large saucepan. Cut the vanilla bean in half and then split it open lengthwise. Scrape vanilla seeds from bean into saucepan and then add vanilla bean to the saucepan. Bring cream mixture to a simmer over medium heat, whisking until sugar dissolves. Remove from heat; cover and let steep for 10 minutes.
3. Add gelatin. Bring cream mixture back to a simmer. Add gelatin mixture and stir until dissolved. Strain mixture through a fine-mesh strainer; discard solids.
4. Chill. Divide strained cream mixture among six 3/4-cup ramekins. Chill uncovered until panna cotta is set, at least 6 hours.

For the marmalade

1. Cook marmalade. Combine lemon zest, lemon juice and sugar in a heavy, small saucepan. Bring to a boil, stirring occasionally, and boil for 5 to 10 minutes or until mixture thickens slightly and reaches 220 degrees.
2. Chill. Remove from heat and transfer to a small lidded glass container or jar. Refrigerate at least 2 hours or until cool and thickened.

To serve

1. Serve. Bring marmalade to room temperature so that is pourable. Top each panna cotta with a spoonful of marmalade and serve.