Preheat oven to 350F and line 2, 12 cups muffin tins with cupcakes liners. Set aside.
Sift the two kinds of flours, baking powder and salt in a bowl and set a side.
In a large measuring cup, combine the milk, half and half and vanilla extract. Mix to combine and set a side.
In a bowl of a standing mixer, beat the butter and sugar together until smooth and creamy, 1-2 minutes.
Add the eggs, one egg at a time waiting for each egg to completely incoporate before adding the next.
Add the dry ingredients in 3 parts alternating with the milk mixture, starting and finishing with the dry ingredients.
Once you added the last portion of the dry ingredients, turn the mixer off and using a rubber spatula stir in the remaining ingredients that may not have been fully incorporated. Allow the batter to rest for 15 minutes.
Divide the batter between the cupcakes liners 3/4 full and bake for about 20 minutes or just until the top of the cupcakes are firm and a cake tester comes out clean from the center of the cupcakes.
In a medium bowl, place the flour and stir in 4 TBS of milk. Mix the milk and flour until you have a smooth pasty mix. Add the remaining of the milk, 2TBS at a time and mix for a smooth mix.
Place the flour and milk mix in a medium pan on top of the stove, medium heat while constantly stirring. Once the mix starts to boil, lower heat to low and keep stirring for 30 seconds more. Remove from heat, you should have a thick mix.
Stir in half of the grape juice zest, pour it to a medium bowl and cover with a plastic wrap. Allow to reach to room temperature.
In a bowl of a standing mixer fitted with the guitar attachment, place the butter and sugar and beat on medium speed until you have a light and fluffy mix. Add the flour and milk mix and beat for 5 minutes longer. Add the remaining of the grapefruit zest and extract and beat to combine.
Once the cupcakes are cooled, frost the cupcakes with the buttercream.