• Preheat the oven to 350F / 165C. Grease and line 2 x 9” round cake tins
• Cream the butter and sugar together in a large bowl until light and fluffy. Add the eggs, one at a time and beating each one in well. Stir in the mashed potato and vanilla.
• In a separate bowl, whisk the flour, cocoa, baking powder and salt. Alternating, add ⅓ of the flour to the egg mixture and gently combine, then ⅓ of the buttermilk mixture. Repeat until all combined.
• Pour the mixture into the prepared tins and bake for 35 minutes or until an inserted skewer comes out clean. Cool for 10 minutes in the tins and then turn out onto a wire rack to cool completely before icing.

For the Chocolate Frosting:

• Melt the chocolate in a microwave or in a bowl over a pot of simmering water (but not touching the water). Allow the chocolate to cool slightly.
• In a large bowl, beat the butter until soft and fluffy. Beat in the powdered sugar, a little at a time. Beat in the vanilla and then gently stir in the melted chocolate. Store in the fridge until ready to use. If too thick, add a teaspoon of milk to loosen. Frost completely cooled cake as desired.

For the Tahini Drizzle:

• Mix the tahini and icing sugar together until thick but still runny. Drizzle over the frosted cake.