Grease and line a 24cm x 24cm baking tin with baking paper. Try to get the baking paper as flush up against the tin as possible by folding in some edges of the paper.

Place the chocolate, butter and condensed milk in a medium sized heatproof bowl. Half fill a small saucepan with water and place the bowl on top, ensuring the water doesn’t touch the bowl. Turn the heat to low-medium and allow the mixture to melt. Stir intermittently until it is completely melted and combined. It should be chocolate in colour and have a thick, shiny appearance.

While the chocolate mixture is melting, crush the biscuits roughly in a large bowl. You can use your hands to break them up or the end of a rolling pin. I prefer to keep lots of bigger chunks because they tend to continue breaking down as you stir the chocolate through.

Add the cocoa powder and walnuts to the crushed biscuits and stir.

When the chocolate is done, add the sea salt and vanilla bean paste, stirring to combine. Pour the chocolate mixture over the biscuits and use a large spoon to completely incorporate.

When the mixture is well combined, pour it into the lined container and use a spatula to get the top as flat as possible, taking it right to the edges. This is important so that the chocolate topping has an even surface to spread over.

Place the slice in the fridge for at least two hours or until completely set.

Melt the chocolate in the same double boiler method you use before. Once melted, add the oil and stir well to combine.

While hot, pour over the refrigerated slice. Work quickly to spread the chocolate evenly or it will begin to harden.

Return to the fridge for an additional hour or so, or until the chocolate has totally hardened.

Allow the slice to sit on the bench for 15-20 minutes (weather dependent) before slicing. This will allow the chocolate to soften and lessen the likelihood of the chocolate cracking. Store in the fridge or freezer. Keeps well in an airtight container for a number of days.