Prepare the cake mix according to package directions, adding the lemon juice, lemon zest and optional yellow food coloring.
Equally spoon the batter into 18-24 muffin tins, bake according to package directions and allow to cool fully.
LEMON BUTTERCREAM FROSTING:
In the bowl of a stand mixer fitted with a paddle attachment, cream the butter for 3-4 minutes until smooth.
With the mixer on low speed, add the powdered sugar one cup at a time and mix until there are no dry spots remaining.
Add the heavy cream and vanilla extract and start mixing on low speed. Slowly Increase to high for 1-2 minutes.
OPtionally, add the food color last and mix until the color is distributed evenly.
Add to a pastry bag with a large round tip (I used a Wilton 1M).
Generously pipe onto each cupcake and decorate as desired.
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Notes
Cupcakes should be stored in an airtight container at room temperature for up to 5 days. While they will last for this long, cakes are generally best enjoyed on the day of or after baking.