Preheat your oven to 350°F (175°C). Line a cupcake tin with 12 liners and prepare additional liners for 3 more cupcakes if needed.

In a medium bowl, whisk together gluten-free baking flour, baking powder, baking soda, and kosher salt. Set aside.

In a large bowl, blend 4 tbsp of unsalted butter and 1/2 cup of granulated sugar until smooth. Gradually add the remaining 1/2 cup of sugar and 1/4 cup of vegetable oil, mixing until creamy.

Add 2 eggs and 1 tbsp of vanilla extract, mixing until well combined.

Blend half of the dry mixture into the wet mixture, then add in the buttermilk and mix. Finally, add the remaining dry ingredients and mix until smooth.

Fill the lined cupcake tins halfway with the batter.

Bake for 15-18 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cupcakes to cool in the pans for about 5 minutes before transferring them to a cooling rack to cool completely.

In a separate bowl, mix 1 cup of room-temperature unsalted butter and 1/2 cup of cocoa powder until smooth.

Gradually add powdered sugar one cup at a time, mixing well after each addition. Thin with milk as needed, and finish with vanilla extract and a pinch of salt.

Once cupcakes are cool, frost them generously with the prepared frosting and top with rainbow sprinkles.