“Veronica Mars” actress Kristen Bell tweeted a picture of herself on Wednesday night enjoying “This Dessert Saves Lives,” a new $10 offering at Back Bay restaurant Post 390. The chocolaty dish is made with a vanilla, almond, and honey bar made by This Bar Saves Lives, an organization that donates food to children in need for every one of its bars sold. Bell is on the group’s board of advisers. And one of the folks who started the project is Todd Grinnell, who’s from Massachusetts. Bell has been spotted around Boston quite a bit lately, probably because her partner, Dax Shepard, has been filming the Robert Downey Jr. movie “The Judge,” which wrapped over the weekend.