Preheat oven to 350℉. Line muffin tins with desired paper liners and set aside. In a small bowl whisk together flour, baking powder, baking soda and salt. Set aside.
Using an electric mixer or stand mixer fitted with whisk attachment, whisk together sugar, oil and lemon zest until combined. Add one egg at a time, beating well after each. Whisk in buttermilk, lemon juice, and lemon extract. Add flour mixture all at once and whisk until smooth, about 1 minute. Carefully add boiling water with mixer on low and whisk until incorporated and batter is smooth, increasing speed when water has been incorporated, about 1 minute.
Using a 1/4 cup measure, scoop batter into prepared cupcake tins and bake in preheated oven for 15-16 minutes or until a toothpick comes out clean. Remove from oven and allow to cool for 10 minutes before removing cupcakes to a wire rack. Once removed from tins, glaze cupcakes as per directions below.
Combine sugar and lemon juice in a small sauce pan over medium heat and whisk until sugar has dissolved and mixture comes to a boil. Remove from heat and use while still warm.
Brush cupcakes with glaze once they have been removed from the tins to a wire rack. You will have enough glaze to make two passes with the glaze.
In a small pot combine sugar, water and lemon juice. Over medium high heat bring mixture to a boil whisking often until sugar has dissolved. Brush any sugar crystals away from the side of the pot with a pastry brushed dunked in cool water. Once sugar has dissolved, stop stirring and attach a candy thermometer to the side of the pot and continue to boil until mixture reaches 240℉.
Using a stand mixer fitted with a whisk attachment, beat egg yolks until smooth and creamy, 1-2 minutes. When sugar syrup reaches 240℉, slowly pour syrup into egg yolks with mixer running on medium high. Try to hit the side of the bowl with the syrup as you pour. Once all the syrup has been incorporated, continue to whisk until the temperature has dropped to room temperature, about 65-68℉.
Add butter by the tablespoon, making sure each piece is incorporated before adding the next. Once all the butter is added, the buttercream should be light and fluffy. Whisk in lemon extract and food colouring.
French buttercream is a little softer than others and therefore recommended here to dollop each cupcake with approximately 2tbsp and smooth over cupcake.
-Start whisking egg yolks once sugar syrup hits about 210℉, that way they will be ready when syrup hits 240℉.
-Use a sealable bag or two filled with ice and a small amount of water to place around the mixer bowl to help it cool to room temperature faster.