Whisk the nut butter, coconut oil, syrup and salt in a bowl.
Pour half the mixture into another bowl and add the cocoa powder and stir until combined. Alternate pouring the mixture in a square baking pan lined with greaseproof paper. Then use a knife to swirl the 2 mixtures to create a marble effect.
Freeze for 4 hours, then serve immediately or portion up and put back in the freezer for up to 1 month.