In a medium mixing bowl whisk together cake flour, all-purpose flour, baking powder and salt, set aside.
In the bowl of an electric stand mixer fitted with the paddle attachment (or in a mixing bowl using an electric hand mixer) cream together sugar and butter on low speed until combined then increase to high speed and whip until very light and fluffy, about 4 minutes. Scrape down bowl.
Add 1 egg and mix until blended, then mix in second egg and vanilla. Scrape down bowl.
Add 1/3 of the flour mixture and mix on low speed just until combined. Add milk and mix just until combined. Add another 1/3 flour mixture, mix. Add buttermilk, mix. Finish by mixing in last 1/3 of the flour mixture.
Remove bowl from stand mixer and scrape bowl and gently fold mixture to ensure ingredients are evenly incorporated.
Divide batter among prepared muffin cups filling each about 2/3 full.
Bake in preheated oven until toothpick inserted into center comes out clean, about 17 to 22 minutes.
Let cool in pan for just a few minutes then transfer to a wire rack to cool completely.*
For the frosting: In the bowl of an electric stand mixer (or using a hand mixer and a mixing bowl) cream together 1/2 cup unsalted butter and 1/4 cup salted butter until pale and fluffy.
Add powdered sugar, heavy cream and vanilla extract and whip until very pale and fluffy (its fluffiness should start to remind you of whipped cream). Frost over cooled cupcakes.
*For best results use a kitchen scale to measure out ingredients for most accuracy and consistent results. I like this one here.
**I like to cool on wire rack about 30 minutes then transfer to an airtight container to finish cooling so they don't dry.
I used Ateco 827 tip to frost those pictured here. If you want a lot of frosting like what is pictured use 1.5 times the recipe (if you have a super sweet tooth).