Combine the softened butter and sugar in a stand mixer bowl. Beat with the paddle attachment until smooth. Mix in the egg, lemon juice, and vanilla until combined.
Using a small ice cream scoop or ¼ cup measuring cup, divide the batter equally among the lined cupcake tins and fill until about ⅔ full. Bake for 14-16 minutes or until the edges are slightly browned and the center is set.
Combine the softened butter, lemon juice, vanilla, and salt in the mixer bowl. Mix until combined. Slowly stir in about half of the confectioner’s sugar until combined.
Add in the remaining sugar and continue to mix on low until combined. Add a splash more lemon juice if needed if the frosting still looks dry and crumbly.
Transfer the frosting to a piping bag fitted with a large star tip. Once the cupcakes are cool, pipe the frosting on each cupcake. Decorate with a couple of blueberries and some additional lemon zest.