- Preheat the oven to 325 ºF (160 ºC) and line two baking trays with parchment paper.
- Using electric beaters or in a stand mixer fitted with the paddle attachment, beat the butter, sugar and lemon zest on medium-high speed until smooth and lighter in colour, about 2 minutes. Add the eggs one at a time, beating well after each is added. Beat in the vanilla
- In a separate bowl, sift together the flour, baking powder and salt. Whisk the sour cream and lemon juice together in another dish. Add half of the flour to the butter mixture and mix on medium-low speed until blended, Add the sour cream and beat in, scraping the bottom of the bowl. Add the remaining flour and mix until evenly combined. Use a medium ice cream scoop and drop scoopfuls of batter onto the trays, leaving 2-inches (5 cm) between them – take care to level each scoopful before dropping and take care to drop them straight down onto the tray (not an angle) so that they bake up evenly. Bake the cupcake tops for 12-15 minutes, until they lift up easily from the tray and colour on the bottom a little (but not on top. Allow the cupcake tops to cool on the trays before frosting them.
- Using beaters or a stand mixer again with the paddle, beat the butter until smooth. Add 2 cups (260 g) of the icing sugar and beat in (start on low speed and increase to medium) until evenly combined. Add the cream, lemon juice and vanilla and beat in, followed by another 2 cups (260 g) of icing sugar, increasing the speed to medium high after blended in. Depending on how soft the butter was to start (and how warm your kitchen is), you may have to add additional icing sugar. Do not add more until you’ve really whipped the frosting on medium high speed – this does fluff it up and give it structure.
- Spoon the frosting into a piping bag fitted with a large star tip. Pipe the frosting onto each cupcake top and finish with a few sprinkles on each. Let the cupcakes sit uncovered for an hour and then store, unrefrigerated, in an airtight container for up to 3 days.