For the Crust: Preheat the oven to 350. Butter a 10-inch springform pan.
Sift the flour, baking soda and salt into a medium bowl and set aside.
In a large bowl, using an electric mixer on medium speed, beat the butter, brown sugar and white sugar till smoothly blended, about 1 minute.
Add the egg and vanilla and mix till blended, about 1 minute. The mixture may look slightly curdled.
On low speed, add the flour mixture, mixing just till it is incorporated. (I just did this with a spoon.) Mix in the chocolate chips.
Spread the dough over the bottom of prepared pan. Bake just till the top begins to turn golden but the center is still soft, about 17 minutes. Cool the crust on a wire rack.
Make the filling: In a medium saucepan, heat the cream and butter over low heat till cream is hot and butter melts. The mixture should form tiny bubbles and reach about 175 degrees F. (I just put it in a glass measuring cup and put it in the microwave for about a minute.) Do not let the mixture boil.
Remove pan from heat, add chocolate chips and let them sit in hot cream mixture for about 30 seconds to soften, then whisk till all of the chocolate is melted and the filling is smooth. Stir in the vanilla.
Let the filling cool and thicken at room temperature for about 30 minutes.
Leaving a 1-inch edge uncovered, spread the sauce over the crust. Refrigerate for about 30 minutes.
Use a small sharp knife to loosen the sides of the crust from the pan and then remove the sides of the pan. Use a sharp knife to loosen the crust from the bottom of the pan and then a wide metal spatula to slide the tart onto a platter.
Make the topping: In a large bowl, using an electric mixer on medium high speed, beat the cream, powdered sugar and vanilla till firm peaks form. (I added about 3 T of powdered sugar, but others might not want it as sweet.).
Spread the whipped cream over the chocolate filling, mounding it slightly in the center. Top with extra chocolate chips, if desired.
Serve, or cover carefully and refrigerate for as long as overnight. Serve cold.