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1. Preheat the oven to 350°F. Grease and flour a 13 x 9-inch baking pan.
2. For cake, place butter, sugar and 1 tsp. vanilla in the bowl fitted with the kneading/crushing blade. Mix at speed 9 for 3 minutes. Scrape down the sides of the bowl and mix for 1 minute more, adding eggs one at a time. Sift together flour, baking powder and salt. Add half the flour mixture to the bowl; mix at speed 6 for 10 seconds. Add half the milk; mix at speed 6 for 10 seconds. Repeat with remaining flour and milk. Mix at speed 8 for 1 minute. Spread the batter in the prepared pan and bake for 27-33 minutes or until a knife inserted in the center comes out clean. Cool at least 1 hour before frosting.
3. To make the frosting, place butter in the bowl with the kneading/crushing blade. Launch the dessert program at speed 4 for 5 minutes. With 1 minute remaining, add cocoa. At the end of the program, add powdered sugar, milk and 1 tsp. vanilla. Mix at speed 6 for 10 seconds. Scrape frosting to the center and mix for another 10 seconds. Spread over top of the cooled cake.