Lemon zest cupcakes with vanilla frosting

I think I should refer these cupcakes as vanilla cupcakes v3.0. Compared to the first and second cupcake, it's way better than both of them combined! A bit nerve racking to make though because it involves egg whites and folding in. I tend to give up the folding bit after two folds and just stir vigorously, which I know is a big no no.

I can't remember where I got this recipe from but I think I did change it slightly when I was going through the whipping up the egg whites phase - changing most cake recipes, whipping up the egg whites and folding it into the batter. I remember using this recipe when I was back home, using double the recipe and baking it in a 9 inch square tin. The cake appeared dense but when it was actually really light.
I quite like practising piping. Other people's videos on how to pipe make it seem so easy but when you actually have a go, it turns out - ugly. Here's the recipe, double it for a 9 inch cake.

Lemon zest cupcakes with vanilla frosting
makes 10 muffin sized cupcakes

4 oz butter
1/2 cup sugar
1/4 tsp salt
4.5 0z flour
1 tsp baking powder
zest of one lemon
1/2 tsp vanilla extract

3 eggs (separated)
1/2 Tbsp lemon juice

1. Preheat oven to 170 C and prepare paper liners in muffin pan.
2. Cream butter and 1/4 cup sugar till fluffy. Add in the 3 egg yolks, one at a time. Mixing on low speed till combined.
3. Mix in flour, baking powder and salt till combined.
4. Add in lemon and vanilla extract.
5. In a clean, dry mixing bowl, beat egg whites, remaining 1/4 cup sugar and lemon juice with an electric mixer until stiff peaks form.
6. Add the beaten egg whites into the batter in 2 separate additions, each time folding gently with a spatula until just blended.
7. Fill up muffin liners till 2/3s full and bake in preheated oven for 15 minutes or a skewer inserted into the centre comes out clean.

Vanilla frosting

4 oz butter, at room temperature
1.5 cups icing sugar
1/2 tsp vanilla extract
1 Tbsp milk

1. Beat the icing sugar and butter together with a paddle attachment on medium-slow speed until the mixture comes together and is well mixed.
2. Turn the mixer down to slow speed. Combine the milk and vanilla extract in a separate bowl, then add to the butter mixture.
3. Once all the milk has been incorporated, turn the mixer up to high speed.
4. Beat until the frosting is light and fluffy, at least 3 minutes. The longer the frosting is beaten, the fluffier and lighter it becomes.