Heat the oven to 375 degrees. Grease a 15x10x1-inch baking pan. Line the bottom with parchment paper. Grease the parchment paper. In a large bowl, beat the eggs whites with an electric mixer on high speed until foamy. Gradually beat in 1/4 cup of the sugar. Beat until stiff peaks form. Set aside. In another bowl, beat the egg yolks on high speed for 5 minutes or until pale yellow and thick. Gradually beat in the remaining 1/2 cup sugar. Beat in ¼ cup sour cream until blended. Add the flour, cocoa powder, baking powder, baking soda, and salt on low speed until blended. Beat on medium speed until creamy. Pour over the egg whites. Fold in and stir until well blended. Spread evenly in the pan. Bake for 12 to 15 minutes or until the center springs back when tapped. Run a knife around the edges to loosen; let stand for 2 minutes. Meanwhile, lay a smooth kitchen towel on the work surface. Sprinkle generously with the 2 tablespoons of powdered sugar. Turn the cake onto the towel. Remove the paper. Spray another piece of parchment paper with cooking spray. Lay the sprayed side down over the top of the cake. Loosely roll the paper-topped cake up in the towel. Cool on a cooling rack until completely cooled (about 1 hour). In a medium bowl, beat 1/2 cup sour cream, milk, pudding mix and extract until thick and smooth. Fold in the whipped topping and ½ cup crushed candy. Carefully unroll the cake and remove the paper. Spread the filling over the cake. Roll the cake over the filling without including the towel. Place on a serving platter, seam side down and refrigerate while preparing the frosting. Place the chocolate chips in a small bowl. In small saucepan, heat the cream until bubbles start to form around the edges. Pour over the chocolate chips. Let stand for 3 minutes. Stir just until the chocolate is melted. Stir in the corn syrup. Let stand for 10 minutes or until the mixture is thick enough to spread. Cut long strips of waxed paper and tuck under the sides of the cake to protect the serving platter. Spread the frosting over the top and sides of the cake. Sprinkle with 2 tablespoons candy. Refrigerate for 15 minutes or until the frosting is set enough to cut before serving. Store loosely covered in the refrigerator.