Make ahead: pour the milk into a medium sauce pan. Roughly chop 2/3 cup (100g) of the pistachios and add them to the milk.
Bring the milk to a medium boil, over medium heat. Until you notice bubbles on the sides of the pan strating to make thier way to the center of the pan. Remove from heat, cover and set aside to chill to room temperature.
Preheat your oven to 340°F (175°C), and line 12-mold cupcake pan with 10-12 cupcake liners.
Pour the pistachio milk into a sieve over a bowl, discard the pistachios then measure 1/3 cup of the milk.
Mix the sour cream (80ml) and the pistachio milk (80ml) and set aside.
Finley chop the rest of the pistachios (50g). You can use a sharp knife, or a food processor.
In a medium bowl, mix the flour (150g), baking powder (1 1/2 teaspoons) and finely chopped pistachio (50g) and set aside.
Cream the butter (55g) with the sugar (100g) in a large bowl, until light and fluffy. About 2 minutes if using a hand mixer, or 4-5 minutes if using a stand mixer fitted with the paddle attachment. Scrape the bottom and sides of the bowl 1-2 times as you go. Add the oil and mix to incoporate.
With the mixer on medium speed, add the eggs, one at a time, waiting for each egg to completely incorporate before adding the next.
Reduce speed to low (or use a rubber spatula), add the flour mixture in three additions alternating with the milk mixture in two additions starting and ending with the flour mixture.
Divide the batter between the cupcake liners and bake over the middle oven rack for 18-20 minutes. Until a toothpick comes out from the center of the cupcakes with a few moist crumbs.
Remove from the oven and let chill for about an hour before filling and frosting.
Top with the pistachio buttercream and serve.