Preheat your oven to 350 degrees and grease a 9 inch loaf pan. Set aside.
In a large bowl, beat together the butter and sugar until fluffy. Add in the eggs and sour cream, and beat until combined. In a medium bowl, whisk together the flour, baking powder, espresso powder, salt, and cocoa powder.
Add the flour mixture and milk alternating into the egg mixture, beating well between each addition, until completely combined. Fold in the pistachios. Spread the batter into your prepared pan.
Bake for 60-65 minutes, or until a cake tester comes out clean. Cool completely before icing.
In a small bowl, whisk together the powdered sugar, water, and vanilla until smooth. Drizzle over the top of your cake before slicing and serving. Enjoy!