Stir together first 2 ingredients and 1/4 cup sugar; press on bottom and 2 inches up sides of a shiny 9-inch springform pan. Sprinkle 3/4 cup pecans over crust.
Microwave chopped chocolate and 1 cup butter in a large microwave-safe glass bowl at HIGH 1 minute or until melted and smooth, stirring at 30-second intervals.
Whisk flour, next 4 ingredients, and remaining 2 cups sugar into chocolate mixture, whisking until blended. Pour batter into prepared crust.
Bake at 350° for 1 hour to 1 hour and 15 minutes or until a wooden pick inserted in center comes out with a few moist crumbs. Remove from oven, and cool in pan on a wire rack 20 minutes.
Preheat broiler with oven rack on lowest level from heat. Place pie (in pan) on a jelly-roll pan. Toss together both marshmallows; mound on pie, leaving a 1/2-inch border around edge. Broil 30 seconds to 1 minute or until marshmallows are golden brown. Remove from oven, and immediately remove sides of pan. Cool on a wire rack 10 minutes.
Meanwhile, prepare Chocolate Frosting. Drizzle over marshmallows; sprinkle with remaining 3/4 cup pecans.
This is great. It tastes exactly like s'mores. So, if you don't like S'mores, don't make this :) It is a very heavy cake though. The bottom half tastes like a fudgy brownie. It is fairly easy to make as well. The kids loved it. One other note - serve it hot. If you try to serve it cold it will be very difficult to cut, and, it tastes best when the marshmallows are melted, just like a s'more.