Preheat the oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.

In a mixing bowl, combine the lemon cake mix, water, vegetable oil, and eggs. Mix until well combined.

Pour the batter into the prepared cake pan.

Bake for 30 minutes or until a toothpick inserted in the center comes out clean.

Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

In a bowl, beat together the powdered sugar, softened butter, lemon juice, and lemon zest until smooth.

Once the cake is completely cooled, spread the frosting evenly over the top.