Whisk together first 4 ingredients in a large heavy saucepan. Gradually whisk in milk and whipping cream. Cook over medium heat, stirring constantly, 10 to 12 minutes or until mixture thickens slightly. Remove from heat.
Whisk egg yolk until slightly thickened. Gradually whisk about 1 cup hot cream mixture into yolk. Add yolk mixture to remaining cream mixture, whisking constantly. Whisk in vanilla bean paste. Cool 1 hour, stirring occasionally.
Place plastic wrap directly on cream mixture, and chill 8 to 24 hours.
Meanwhile, preheat oven to 350°. Bake pecans in a single layer in a shallow pan 8 minutes or until toasted and fragrant, stirring once halfway through. Cool completely (about 30 minutes).
Pour chilled cream mixture into freezer container of a 1 1/2-qt. electric ice-cream maker, and freeze according to manufacturer's instructions. (Instructions and time may vary.) Before transferring ice cream to an airtight container for further freezing, stir in pecans and chocolate. Garnish, if desired.