1Heat oven to 350°F. Grease 9x1-inch tart pan with removable bottom with shortening. Wrap outside bottom and side of pan with foil to prevent leaking. In 1-quart saucepan, heat bittersweet chocolate and 1/3 cup butter over low heat, stirring frequently, until melted; cool slightly.
2In large bowl, mix granulated sugar, flour and eggs with spoon until well blended. Stir in chocolate mixture. Pour into pan. Bake 30 to 35 minutes or until edges are set. Cool completely on wire rack, about 1 hour.
3In 1-quart saucepan, heat unsweetened chocolate and 1 teaspoon butter over low heat, stirring constantly, until melted. Remove from heat. Stir in powdered sugar and 2 tablespoons water until smooth. Stir in additional boiling water, 1/2 teaspoon at a time, until spreadable. Spread warm glaze over tart.
4In small microwavable bowl, microwave white chocolate and oil on Medium (50%) 20 to 30 seconds; stir. Drizzle over warm glaze. Let stand until glaze is set. Remove foil and side of pan before serving.