Preheat the oven to 160°C fan / 180°C conventional. Grease and line a 2lb loaf tin with baking paper.
In a small bowl, dissolve the instant coffee in 1.5 tbsp boiling water. Place in the fridge to cool.
In a large bowl, cream the butter and sugar together until light and fluffy. Beat in the eggs one at a time, adding one tablespoon of the flour with each to stop the mixture from curdling. Fold in the remaining flour. Fold in the cooled coffee until the cake mixture is smooth and combined.
Spread half of the mixture into the loaf tin. Microwave the Nutella for 30 seconds to melt it slightly then drop spoonfuls on top of the layer of cake mixture.
Spoon the remaining cake mixture on top and spread it out to conceal the Nutella underneath. Use a large skewer to swirl the layers together.
Bake for 65-70 minutes until risen, golden and the sponge springs back when lightly pressed in the middle. Leave to cool in the tin for 15 minutes, then lift out and place on a wire rack to cool completely.