Immediately, in 4-quart saucepan, heat 1/2 cup butter, the milk and 1/4 cup baking cocoa over medium heat, stirring frequently, until butter is melted and mixture is well blended. Remove from heat. Beat in powdered sugar and 1 teaspoon vanilla with whisk until smooth. Stir in pecans. Pour over hot cake, spread evenly. Cool completely, about 2 hours.