Preheat the oven to 350°F (180°C). Line a mini muffin tin with paper liners.
In a medium bowl, whisk together the flour, sugar, baking powder, and salt.
In a large bowl, using an electric mixer, beat the butter until creamy. Add the eggs one at a time, beating well after each addition.
Beat in the lemon zest, lemon juice, and vanilla extract.
Gradually add the dry ingredients to the wet ingredients, beating until just combined.
Divide the batter evenly among the mini muffin cups.
Bake for 12-15 minutes, or until a toothpick inserted into the center of a cake comes out clean.
Allow the cakes to cool completely in the pan.
To make the lemon buttercream frosting, beat the butter until creamy. Gradually add the powdered sugar, beating until smooth and creamy.
Beat in the lemon zest, lemon juice, and salt.
Add the heavy cream or whole milk, starting with 2 tablespoons and adding more as needed, until the frosting reaches the desired consistency.
To make the lemon drop filling, whisk together the powdered sugar, lemon juice, and salt until smooth.
To assemble the mini lemon drop cakes, use a small cookie scoop or a spoon to create a hole in the center of each cake.
Fill the hole with a small amount of lemon drop filling.
Top the cake with a dollop of lemon buttercream frosting.