Preheat the Oven: Set your oven to 350°F (175°C) and line a muffin tin with cupcake liners.
Mix Wet Ingredients: In a medium bowl, whisk together melted butter, eggs, sour cream, milk, lemon juice, and sugar until smooth.
Prepare Dry Ingredients: In a separate bowl, sift together flour, baking soda, baking powder, and salt.
Combine Wet and Dry: Gradually add the wet ingredients to the dry ingredients and mix until just combined.
Fill Cupcake Liners: Scoop the batter into the cupcake liners, filling each about 2/3 full.
Bake: Bake for 18-20 minutes or until a toothpick comes out clean. Let cupcakes cool.
Prepare the Frosting: Beat butter until creamy, then gradually add powdered sugar, salt, lemon zest, and juice. Add vanilla extract and beat until smooth.
Decorate: Pipe or spread the lemon buttercream on the cooled cupcakes.