1. Preheat oven to 350°F degrees.
2. Add cupcake liners to a regular muffin pan. Set aside.
3. To a large bowl add dry ingredients: flour, sugar, baking powder and salt, whisk to combine.
4. To another large bowl (or the bowl of a stand mixer), add milk, oil, lemon juice, lemon zest, and eggs. Whisk on medium speed until fully combined, about 1 minute.
5. Add the dry ingredients to the wet ingredients and beat until well combined. Stop once to scrape the sides and bottom of the bowl with a spatula.
6. With the mixer on low speed, slowly add the water and after that the food coloring if using. Mix just until combined. Scrape down the sides of the bowl as needed.
7. Fill the cupcake liners with batter just a little over half.
8. Bake for 15-20 minutes, or until a toothpick comes out with a few moist crumbs NOT raw batter. Start checking on the cupcakes at the 15-minute mark. If you are baking mini cupcakes, they may be done in 9-12 minutes.
9. Once done, remove the tray from oven and allow to cool for 3-5 minutes, then remove cupcakes to a cooling rack to finish cooling.

Frosting:

1. Add butter to a large mixer bowl and mix until smooth.
2. Add 4 cups of powdered sugar and mix until smooth.
3. Add the vanilla extract, gel food coloring and lemon juice, mix until smooth.
4. Add the remaining 4 cups of powdered sugar and mix until smooth. If the buttercream is too soft, add more sugar, 1/2 cup at a time.
5. Fix a piping bag with a large star tip or any other piping tip that you like.
6. Pipe the buttercream onto the cupcakes and garnish with lemon slices.
7. Refrigerate cupcakes at least one hour before serving.
8. Store in the refrigerator for 3-4 days.