Make the tart: Preheat oven to 325F. Lightly grease and flour a 12-in tart pan. Set aside.
In a small saucepan over medium heat, melt butter and chocolate together until smooth, stirring occasionally. Meanwhile, in a medium bowl, whisk eggs, espresso, vanilla, and salt until smooth. Whisk in melted chocolate mixture.
Pour batter into prepared pan. Bake tart at 325F 20-25 minutes or until tart springs back when lightly touched. Cool tart completely on a wire cooling rack. (middle will collapse a little bit as it cools--this is ok)
Make the sauce: Combine all sauce ingredients in a small saucepan. Bring to a boil over medium-high heat, then reduce heat to medium and let sauce simmer until thickened. Let cool.
Once sauce and tart is cooled, spoon sauce into the middle of tart. Drizzle tart with white chocolate and let set 10 minutes. Slice tart into wedges and enjoy!