In a large bowl, beat 1/2 cup butter with an electric mixer on medium to high speed for 30 seconds. Beat in the 2/3 cup sugar. Beat on medium-high speed about 3 minutes or until light and fluffy. Sprinkle cocoa over top of mixture. Beat on low speed until blended. Add egg yolk, the 1 teaspoon vanilla, and the chocolate extract; mix well. Beat in flour.
Pat dough mixture into bottom and up side of a 10-inch tart pan with removable bottom. Generously prick bottom and side with the tines of a fork. Bake for 20 to 25 minutes or until dry and tiny blisters appear on the bottom (crust may puff slightly but will settle as it cools). Cool in pan on a wire rack.
For the caramel filling:
In a large saucepan, combine the 2 cups sugar, the water, and corn syrup. Do not stir. Cook over medium-high heat, swirling pan often, for 12 to 15 minutes or until syrup is dark amber in color. Remove from heat. Very carefully whisk in the 1/2 cup cream, 1/2 cup butter, and the 1/2 teaspoon vanilla. (The mixture will sputter and pop!) Continue whisking until perfectly smooth. Pour immediately into tart shell. Allow to stand about 1 hour or until caramel is set, but not firm.
For the ganache topping:
Place chocolate in a medium bowl. In a small saucepan, bring the 3/4 cup cream to boiling. Pour immediately over chocolate (do not stir). Allow to stand for 5 minutes, then whisk until perfectly smooth. Carefully pour warm ganache topping over tart. Cover loosely; chill overnight.
Remove side of pan. Let stand at room temperature for 30 minutes. Cut in wedges. If desired, sprinkle wedges with sea salt. Makes 12 servings.