Step 1: Prepare the Raspberry Filling

1. In a medium saucepan, combine raspberries, sugar, and lemon juice.
2. Cook over medium heat until the raspberries break down and release their juices, about 5-7 minutes.
3. In a small bowl, mix cornstarch and water to create a slurry.
4. Stir the cornstarch mixture into the raspberry mixture and continue cooking for 1-2 minutes until thickened.
5. Remove from heat and strain through a fine-mesh sieve to remove seeds (optional but recommended).
6. Allow the filling to cool completely in the refrigerator while preparing the cupcakes.

Step 2: Make the Pistachio Cupcakes

1. Preheat your oven to 350°F (175°C) and line a 12-cup muffin tin with cupcake liners.
2. In a food processor, pulse ½ cup of pistachios until finely ground to make pistachio flour. Be careful not to over-process.
3. In a medium bowl, whisk together all-purpose flour, pistachio flour, baking powder, and salt.
4. In a large bowl, cream together the butter and sugar until light and fluffy, about 3-4 minutes.
5. Add eggs one at a time, beating well after each addition.
6. Mix in vanilla extract, almond extract, and pistachio extract (if using).
7. Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk (begin and end with dry ingredients).
8. Fold in the chopped pistachios.
9. Divide the batter evenly among the cupcake liners, filling each about ⅔ full.
10. Bake for 18-20 minutes or until a toothpick inserted into the center comes out clean.
11. Allow the cupcakes to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.

Step 3: Make the Pistachio Buttercream

1. In a large bowl, beat the butter until creamy and smooth, about 2 minutes.
2. Gradually add the powdered sugar, one cup at a time, mixing well after each addition.
3. Add the pistachio paste, salt, and vanilla extract, beating until fully incorporated.
4. Add heavy cream one tablespoon at a time until desired consistency is reached.
5. If desired, add a small amount of green food coloring to enhance the natural pistachio color.
6. Beat on high speed for 3-4 minutes until light and fluffy.

Step 4: Assemble the Cupcakes

1. Once cupcakes are completely cool, use a small knife or cupcake corer to remove the center of each cupcake, creating a well for the raspberry filling.
2. Fill each cupcake with the cooled raspberry filling.
3. Transfer the pistachio buttercream to a piping bag fitted with your favorite decorative tip.
4. Pipe the buttercream onto each cupcake, covering the raspberry filling.
5. Garnish with additional chopped pistachios and fresh raspberries.