Preheat the oven to 350°F (175°C). Grease and line a baking pan with parchment paper.

In a mixing bowl, whisk together melted butter and sugar until smooth.

Add eggs and vanilla extract, mixing until well combined.

In another bowl, sift together flour, cocoa powder, baking powder, and salt.

Gradually add dry ingredients to the wet mixture, stirring until just combined.

Fold in chopped pistachios.

Pour the batter into the prepared baking pan and spread evenly.

Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean.

Allow brownies to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

In a saucepan, heat the heavy cream over medium heat until it begins to simmer.

Remove from heat and add chopped white chocolate, stirring until melted and smooth.

Stir in pistachio paste and salt until fully incorporated.

Let the ganache cool slightly before pouring over the cooled brownies.

Once the ganache has set, cut the brownies into squares and serve.