Preheat oven to 350°F. Mix lemon cake mix with water, oil, and eggs per package directions. Pour into a greased 9x13-inch pan and bake 28–33 minutes, until a toothpick comes out clean.
Cool cake in the pan for 15 minutes. Poke holes all over with a fork or skewer. Whisk 1 cup lemonade concentrate with 1 cup powdered sugar. Pour slowly over the cake, letting it soak in.
Let the cake cool completely. Cover and refrigerate for at least 1 hour to set.
Beat softened cream cheese and butter until fluffy. Add 3–4 cups powdered sugar, 3 tbsp lemonade concentrate, and a pinch of salt. Mix until smooth and creamy. Adjust sugar as needed.
Spread frosting over the chilled cake. Slice and serve cold for the best flavor and texture.