Preheat oven to 350°F. Prepare mini tart pans with vegetable cooking spray; arrange on sheet pan.
In large bowl, stir together melted butter, sugar, vanilla and salt. Add flour; stir until just combined. Divide dough into tart pans and evenly press into the bottom and up the sides. Refrigerate 30 minutes. With fork, prick the shells all over. Bake 20-22 minutes, or until lightly golden brown. Cool completely.
Place chopped chocolate in large bowl. In microwave-safe bowl, heat cream and 4 teaspoons butter for 1-2 minutes or until steaming. Pour over chocolate; let stand 5 minutes. Stir to melt chocolate and completely combine. If the chocolate is not fully melted, return it to the microwave for 15-30 seconds at 50% power. Pour into tart shells; chill until set, about 2 hours. Garnish with sea salt and an orange slice.