Preheat Oven and Prepare Pans:
Preheat your oven to 350°F (175°C). Grease and flour a round 8-inch cake pan or line the bottom with parchment paper.

Make the Cake Batter:
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a large bowl, beat the softened butter and sugar together until light and fluffy, about 2-3 minutes. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract and melted white chocolate.

Combine Dry and Wet Ingredients:
Gradually add the dry ingredients to the wet mixture, alternating with the milk, and mix until smooth. Fold in the chopped pistachios.

Bake the Cake:
Pour the batter into the prepared cake pan and smooth the top. Bake for 25-30 minutes or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely.

Make the White Chocolate Ganache:
In a heatproof bowl, combine the chopped white chocolate, heavy cream, and butter. Heat over a double boiler or in the microwave in 20-second intervals, stirring after each, until smooth and fully combined. Let the ganache cool slightly before pouring over the cooled cake.

Assemble and Garnish:
Once the cake is cool, pour the white chocolate ganache over the top, letting it drip down the sides. Decorate with whole pistachios for a festive touch.

Serve and Enjoy:
Allow the ganache to set before slicing and serving. Enjoy the rich, indulgent flavor of this white chocolate pistachio cake!