In small bowl, sift together flour and cocoa, set aside.
In large bowl, beat together butter, sour cream, egg, sugar and vanilla. Slowly beat in flour. Beat on medium for 2 minutes. Pour into pan. Bake 30-35 minutes, or until toothpick inserted in center comes out clean. Cool completely.
Make glaze: Combine cream, sugar, butter, corn syrup and chocolate chips. Cook over medium heat stirring constantly, until mixture boils. Continue to cook stirring constantly for 5 minutes. Remove from heat and cool 10 minutes. Stir in chopped nuts and vanilla.
Place cake on serving platter. Pour glaze evenly over cake, allowing some to run down the sides. Arrange pecan halves on top. Refrigerate until glaze is firm, about 1 hour.