Prepare the pan and oven

• Preheat the oven to 175 °C.
• Grease and line the loaf pan with baking paper.

Cream the butter, sugar and pistachio cream

• Beat the softened butter with the granulated sugar until pale and fluffy.
• Beat in the pistachio cream until smooth.

Add the eggs and vanilla

• Add the eggs one at a time, beating well after each addition.
• Stir in the vanilla bean paste.

Make the batter

• Whisk the flour, baking powder and salt together.
• Fold the dry ingredients into the butter mixture alternately with the buttermilk, until just combined.

Create the chocolate swirl

• Divide a third of the batter into a bowl and fold in the cocoa powder.
• Spoon the plain pistachio batter and the chocolate batter alternately into the loaf pan.
• Swirl gently with a knife or spatula to marble.

Bake the loaf

• Bake until a skewer inserted in the centre comes out clean.
• Let the cake cool in the pan for 10 minutes, then turn out onto a cooling rack.

Make the pistachio frosting

• Beat the powdered sugar with a little pistachio cream and a splash of buttermilk until silky and spreadable.

Finish and serve

• Spread the pistachio frosting over the cooled cake.
• Sprinkle with the crushed toasted pistachios.