To prepare the cupcakes, preheat the oven to 350°F/180°C. Place paper cupcake liners in a 12-c. cupcake pan.
In a mixing bowl, combine the flour, sugar, baking powder and salt, stirring together to mix thoroughly.
In the bowl of an electric mixer fitted with a flat paddle, combine the egg yolks, vegetable oil, lemon juice, milk and lemon zest and mix together on low speed for 30 seconds.
Gradually add the flour/sugar mixture to the egg mixture and mix together in short blasts to incorporate the dry ingredients with the egg mixture. Scrape down the sides and mix for just about 15 seconds to incorporate the ingredients well.
In a separate bowl using clean, dry beaters, beat the egg whites and the cream of tartar together until stiff peaks form.
Using a spatuala, fold in the egg whites very gently to the flour mixture to create a light batter. Be careful not to overwork the batter.
Pour the batter into the cupcake cups and bake for about 30 - 35 minutes or until the cake springs back when lightly touched.
Allow the cupcakes to cool in the pan for about 10 minutes before removing to cool completely on wire racks before frosting.
To prepare the frosting, cream the butter, cream cheese, lemon extract and salt together until smooth in a medium mixing bowl.
Start adding the sugar gradually, allowing the butter mixture and sugar to cream together before adding more, scraping down the sides as you go. The mixture should look a little dry at this point.
Start adding the liquid 1 Tbsp. at a time and beat on high speed until the frosting is smooth and spreadable.
Smooth the frosting on cool cupcakes with a spatula or pipe out the frosting using a pastry bag and a large star-shaped or circle piping tip to create a tall, swirl pattern.
Sprinkle the cupcakes with fresh lemon zest.