Prepare the Honey Cake Layers: Preheat your oven to 350°F (175°C) and grease two 9-inch round cake pans. Line them with parchment paper for easy removal. In a mixing bowl, whisk together the sour cream, eggs, honey, oil, and vanilla extract until smooth. In a separate bowl, sift the flour, baking soda, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Divide the batter evenly between the two prepared pans. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Allow the cakes to cool completely.

Make the Pistachio Buttercream: Process the pistachios in a food processor until finely ground. Add the heavy cream and pulse until a smooth paste forms. In a stand mixer, beat the softened butter until light and fluffy. Add the pistachio paste and vanilla extract, and continue beating. Gradually add the confectioner's sugar, one cup at a time, beating until the frosting is smooth and creamy.

Assemble the Cake: Level the cake layers with a serrated knife if needed. Place one cake layer on a serving platter and spread a generous amount of pistachio buttercream on top. Sprinkle half of the diced pistachios evenly over the buttercream, then drizzle with honey. Place the second cake layer on top and repeat with the remaining buttercream, pistachios, and honey.

Decorate and Serve: Smooth the buttercream around the sides and top of the cake, and garnish with extra pistachios or a drizzle of honey for a polished finish. Chill the cake for at least 1 hour before serving to allow the layers to set.