Coat an 8-cup soufflé dish with nonstick cooking spray. Line bottom with wax paper; spray paper.
Beat together the pound cake mix, buttermilk, butter, eggs, lemon juice and zest on low speed for 30 seconds; increase speed to medium and beat for 2 to 3 minutes or until well blended and smooth. Spread batter into prepared dish. Cover dish tightly with foil and place in a 5-quart oval slow cooker bowl. Add 4 cups boiling water around soufflé dish. Cover and cook on HIGH for 4 hours. Cake is done when a toothpick inserted in the center tests clean and cake springs back when touched.
Remove dish to a rack and cool uncovered for 10 minutes. Run a thin knife around edge of cake and invert onto wire rack; turn right side up to cool completely.
In medium-size bowl, beat confectioners' sugar, butter, milk and lemon extract until smooth. Spread over cake.