Step 1: Preheat oven and prepare pans. Preheat your oven to 350°F (175°C). Grease and flour two 8-inch round cake pans or line with parchment paper. This ensures your cake layers release beautifully.
Step 2: Toast and grind pistachios. Spread shelled pistachios on a baking sheet and toast in the preheated oven for 5-7 minutes, until fragrant. Let cool completely, then grind finely in a food processor.
Step 3: Whisk dry ingredients. In a medium bowl, whisk together the all-purpose flour, ground pistachios, baking powder, baking soda, and salt. Set aside.
Step 4: Cream butter and sugar. In a large bowl, using an electric mixer, cream together the softened unsalted butter and granulated sugar until light and fluffy, about 3-5 minutes.
Step 5: Add eggs and vanilla. Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
Step 6: Alternate dry and wet ingredients. Gradually add the dry ingredient mixture to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined; do not overmix.
Step 7: Divide and bake. Divide the batter evenly between the prepared cake pans. Bake for 25-30 minutes, or until a wooden skewer inserted into the center comes out clean.
Step 8: Cool cakes. Let the cakes cool in the pans for 10 minutes before inverting them onto a wire rack to cool completely.
Step 9: Prepare raspberry syrup. In a small saucepan, combine fresh raspberries, granulated sugar, and water. Bring to a gentle simmer over medium heat, stirring occasionally, until the sugar dissolves and the raspberries soften, about 5 minutes. Strain through a fine-mesh sieve if you prefer a seedless syrup. Let cool completely.
Step 10: Prepare mascarpone buttercream. In a large bowl, beat the softened unsalted butter with an electric mixer until light and fluffy, about 3 minutes.
Step 11: Add powdered sugar. Gradually add the powdered sugar, one cup at a time, beating until smooth after each addition.
Step 12: Incorporate mascarpone. Add the cold mascarpone cheese and vanilla extract. Beat on low speed until just combined, then increase to medium-high and beat until light and fluffy, being careful not to overmix.
Step 13: Assemble the cake. If necessary, level the cooled cake layers with a serrated knife. Place one cake layer on your serving plate. Brush generously with half of the raspberry syrup.
Step 14: First buttercream and raspberry layer. Spread about 1/3 of the mascarpone buttercream over the first cake layer. Arrange a layer of fresh raspberries over the buttercream.
Step 15: Second cake layer. Place the second cake layer on top. Brush with the remaining raspberry syrup.
Step 16: Frost and garnish. Spread the remaining mascarpone buttercream over the top and sides of the cake. Decorate with additional fresh raspberries and chopped pistachios.
Step 17: Chill and serve. Refrigerate the cake for at least 30 minutes before serving to allow the frosting to set. Enjoy!