Follow these detailed steps carefully to ensure a perfect Pistachio Rose Cake every time. The process is broken down into making the cake layers, preparing the frosting, and assembly.

Part 1: Baking the Pistachio Cake Layers

1. Preheat and Prepare: Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans. For extra insurance against sticking, you can also line the bottom of each pan with a circle of parchment paper.
2. Grind the Pistachios: Place the 1 ½ cups of shelled, unsalted pistachios into a food processor. Pulse in short bursts until they are finely ground, resembling a coarse flour or fine meal. Be careful not to over-process, or you will end up with pistachio butter. A good tip is to add a tablespoon of the measured all-purpose flour to the food processor to help absorb oils and prevent clumping.
3. Combine Dry Ingredients: In a large bowl, whisk together the all-purpose flour, the freshly ground pistachios, granulated sugar, baking powder, baking soda, salt, and optional ground cardamom. Whisking thoroughly ensures all the leavening agents and flavors are evenly distributed, which is key to a uniform cake.
4. Cream Butter and Sugar: In a separate large bowl using a stand mixer fitted with a paddle attachment or a hand mixer, beat the softened butter on medium-high speed for about 2-3 minutes until it’s light, pale, and fluffy. This step incorporates air into the butter, which is the foundation of a light cake texture.
5. Add Eggs and Flavorings: Add the room temperature eggs one at a time, beating well after each addition until fully incorporated. Scrape down the sides and bottom of the bowl as needed to ensure everything is mixed evenly. After the last egg is mixed in, beat in the vanilla extract and the 2 teaspoons of rose water.
6. Alternate Wet and Dry Ingredients: With the mixer on low speed, add the dry ingredient mixture in three additions, alternating with the buttermilk in two additions. Begin and end with the dry ingredients (Dry, Wet, Dry, Wet, Dry). Mix only until just combined after each addition. This method prevents the gluten from overdeveloping and keeps the cake tender. Do not overmix the batter at this stage. A few small lumps are perfectly fine.
7. Divide and Bake: Evenly divide the batter between your two prepared cake pans. Use a spatula to smooth the tops. Bake in the preheated oven for 30-35 minutes, or until a wooden skewer or toothpick inserted into the center of the cakes comes out clean or with a few moist crumbs attached. The tops should be golden brown and spring back lightly when touched.
8. Cool the Cakes: Let the cakes cool in their pans on a wire rack for about 15-20 minutes. Then, carefully run a knife around the edges of the pans and invert the cakes onto the wire rack to cool completely. It is crucial that the cakes are 100% cool before you begin frosting, otherwise, the buttercream will melt.

Part 2: Preparing the Rose Buttercream and Assembling the Cake

1. Beat the Butter: In the clean bowl of your stand mixer with the paddle attachment (or using a hand mixer), beat the 1 ½ cups of softened butter on medium-high speed for 5-7 minutes. This may seem like a long time, but it’s the secret to an incredibly light, fluffy, and less greasy-tasting American buttercream. The butter should become very pale and airy.
2. Add Sugar and Liquid: Reduce the mixer speed to low. Gradually add the sifted powdered sugar, one cup at a time, alternating with the heavy cream or milk.
3. Whip to Perfection: Once all the sugar and liquid are incorporated, add the 1 ½ tablespoons of rose water and the ¼ teaspoon of salt. Increase the mixer speed to medium-high and whip for another 3-5 minutes until the frosting is exceptionally smooth, light, and fluffy. If you’re using food coloring, add it now and mix until the color is uniform.
4. Adjust Consistency: Check the frosting’s consistency. If it’s too stiff, add another tablespoon of cream. If it’s too thin, add more sifted powdered sugar, a quarter cup at a time, until it reaches a thick, spreadable consistency.
5. Assemble the Cake: Place one of the completely cooled cake layers on your serving plate or cake stand. Spread a generous layer of rose buttercream over the top, about 1 cup. Carefully place the second cake layer on top.
6. Frost the Exterior: Use an offset spatula to apply a thin layer of frosting all over the top and sides of the cake. This is called a “crumb coat” and it traps any loose crumbs. Place the cake in the refrigerator for 20-30 minutes to set this layer.
7. Final Frosting and Garnish: Once the crumb coat is firm, apply the remaining frosting to the top and sides, using your spatula to create a smooth finish or decorative swirls. Just before serving, sprinkle the top with the chopped pistachios and edible dried rose petals for a breathtaking presentation.