Preheat Your Oven: Set your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans to ensure a smooth release after baking.
Mix Dry Ingredients: In a medium bowl, combine the flour, baking powder, baking soda, and salt. Whisk until well blended.
Cream the Butter and Sugar: In a large bowl, beat the softened butter and granulated sugar until light and fluffy. This step is key for a light cake!
Add Eggs and Vanilla: Incorporate the eggs one at a time, ensuring each is well blended before adding the next, followed by the vanilla extract.
Alternate Dry and Wet Ingredients: Add the dry ingredients into the butter mixture alternately with the sour cream and whole milk, starting and ending with the flour mixture. Stir in the chopped pistachios and white chocolate chips last.
Bake the Cake: Divide the batter evenly between the prepared pans and smooth the tops. Bake for 25-30 minutes or until a toothpick comes out clean. Allow the cakes to cool in the pans for 10 minutes before transferring them to wire racks.
Prepare the Frosting: In a bowl, beat the softened butter until creamy, then gradually add the powdered sugar. Mix in the heavy cream and vanilla, beating until fluffy.
Frost the Cake: Once the cakes are completely cool, layer them with frosting. Spread frosting on top of the first layer, add the second layer, and cover the entire cake with a generous amount of frosting. Finish by garnishing with the chopped pistachios.