Pulse first 3 ingredients in a food processor 3 or 4 times or until combined. Add butter, and pulse 5 or 6 times or until crumbly. With processor running, gradually add 2 Tbsp. water, egg yolk, and vanilla; process until dough forms a ball and leaves sides of bowl, adding more water if necessary. Cover and chill 1 1/2 hours.
Roll dough to a 12" circle on a lightly floured surface. Fit pastry in a 9" tart pan with removable bottom. Trim off excess pastry, allowing edges to overhang 1/2"; fold in overhang against inside edge of pan to form double-thick sides. Pierce bottom of pastry with a fork; freeze 20 minutes. Bake at 375° for 30 minutes or until lightly browned. Cool completely in pan on a wire rack.
Whisk together whipping cream and next 3 ingredients in a medium saucepan. Bring to a simmer over medium-low heat. Add chocolate, and whisk until smooth. Pour filling into baked crust. Refrigerate 2 hours or until firm.
Whisk together 1 cup water, 1 cup sugar, and, if desired, 1/2 tsp. curry powder in a large heavy skillet. Bring to a simmer over medium-high heat, stirring until sugar dissolves. Add orange slices to pan. Reduce heat to medium-low; simmer 25 to 30 minutes, turning slices occasionally, or until orange slices are translucent. Remove pan from heat; allow orange slices to cool in syrup.
Arrange orange slices on top of tart before serving. Spoon remaining syrup over each serving.
Fix It Faster: Omit the homemade crust (first 7 ingredients), and use 1/2 of a 15-oz. package of refrigerated piecrusts. Unroll piecrust and fit into pan. Prick with a fork. Bake at 375° for 18 minutes or until golden. Cool completely and proceed with filling.