Preheat your oven to 350°F (175°C) and grease and flour two 6- or 7-inch cake pans thoroughly.

In a small bowl, combine milk with apple cider vinegar or lemon juice and let sit for 5 minutes.

Blend ground pistachios with granulated sugar in a food processor until it resembles a fine powder.

In a large mixing bowl, whisk together all-purpose flour, baking powder, salt, and pistachio-sugar mixture.

In a separate bowl, whisk together the prepared buttermilk, olive oil, vanilla extract, and almond extract.

Gently fold the dry mixture into the wet ingredients until just combined.

Divide the batter evenly between the prepared pans and bake for 25-30 minutes.

Let the cakes cool in the pans for 10 minutes before transferring to a wire rack to cool completely.

Beat softened unsalted butter until creamy, then add blackberry preserves and powdered sugar until combined.

Level the first cake layer, spread frosting and blackberry preserves between layers, frost the top and sides, and decorate with fresh blackberries and chopped pistachios.