Preheat your oven to 350°F (175°C). Line your cupcake pan with cupcake liners.
In a large mixing bowl, whisk together the cake flour, all-purpose flour, baking powder, and kosher salt until well combined.
In another bowl, using an electric mixer, beat the unsalted butter and granulated sugar together until light and fluffy, about 3-5 minutes.
Add the eggs one at a time, mixing well after each addition. Then, mix in the vanilla extract.
Gradually add the dry ingredient mixture to the wet ingredients, alternating with the whole milk. Start and end with the dry ingredients, mixing just until combined. Be careful not to overmix!
Spoon the batter into the prepared cupcake liners, filling each about two-thirds full.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Once baked, remove the cupcakes from the oven and let them cool in the pan for about 5 minutes. Then, transfer them to a cooling rack to cool completely.