Preheat oven to 350˚F. Line a muffin tin with paper liners.
Whisk together the flour, sugar, zest, baking powder, and salt. Create a well in the center.
Add the egg, milk, vegetable oil, lemon juice, and extract into the well. Mix until everything is combined. Add the boiling water and whisk until smooth.
Pour batter into paper liners using a measuring cup or trigger scoop. Bake for 15-18 minutes.
Remove cupcakes from tin when cool enough to handle. Frost when cool.
For the Lemon Buttercream Frosting:
Beat the butter for 2 minutes on medium-high until light and fluffy.
On low speed, slowly add the powdered sugar followed by the cream, extract and salt. Increase speed to medium and beat for about 5 minutes or pale in color.
Frost cupcakes, sprinkle with zest, and top with half a lemon slice.