Preheat the oven to 350°F (175°C). Line a muffin tin with cupcake liners.
In a large bowl, mix the cake mix, pudding mix, eggs, sour cream, oil, and water. Use a hand mixer to blend for about 2 minutes.
Scoop the batter into the cupcake liners, filling each about 2/3 full.
Bake for 18-22 minutes, or until a toothpick inserted comes out clean.
Allow the cupcakes to cool completely before frosting.
For the frosting, beat the softened butter, powdered sugar, lemon juice, and zest until smooth and creamy.
Frost the cupcakes with the lemon buttercream and garnish with extra lemon zest or a lemon slice.