Prepare the Cupcakes:

1. Preheat oven to 350°F (175°C) and line a cupcake tin with paper liners.
2. In a bowl, whisk together flour, baking powder, and salt. Set aside.
3. In a separate bowl, cream butter and sugar until light and fluffy.
4. Add eggs one at a time, mixing well after each addition. Stir in vanilla extract, lemon juice, and lemon zest.
5. Alternate adding the flour mixture and milk to the butter mixture, beginning and ending with flour. Mix until just combined.
6. Divide batter evenly among cupcake liners.
7. Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Let cool completely.

Prepare the Lemon Buttercream:

1. Beat butter until creamy. Gradually add powdered sugar and mix until fluffy.
2. Add lemon juice, lemon zest, and vanilla extract. Mix until fully incorporated.
3. Add heavy cream, one tablespoon at a time, until the desired consistency is reached.

Decorate the Cupcakes:

1. Pipe a generous swirl of lemon buttercream onto each cupcake.
2. Garnish with lemon zest, a fresh lemon slice, and an optional mint leaf for a refreshing touch.