1. Preheat the oven to 350°F. Grease a 9-inch round cake pan and line the bottom with parchment paper.

2. In a medium bowl, whisk together the flour, cocoa powder, baking powder, and salt.

3. In a large bowl, beat the butter and sugar until light and fluffy, about 2-3 minutes. Beat in the eggs one at a time, then stir in the vanilla.

4. Gradually add the dry ingredients to the wet ingredients and mix just until combined. Fold in the chopped pistachios.

5. Pour the batter into the prepared cake pan and smooth the top. Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.

6. Allow the cake to cool in the pan for 10 minutes, then invert it onto a wire rack to cool completely.

7. In a medium saucepan, heat the heavy cream over medium heat until it just begins to simmer. Remove from heat and add the chopped chocolate. Let sit for 2-3 minutes, then stir until the chocolate is melted and the mixture is smooth.

8. Pour the chocolate ganache over the top of the cooled cake, allowing it to drip down the sides. Sprinkle with additional chopped pistachios.

9. Refrigerate the cake for at least 2 hours before serving to allow the ganache to set.