Step 1

Grease a 9x5 inch loaf tin and then press a sheet of non-stick baking paper into it, leaving an overhang for easy removal. You can also make this in an 8-inch round springform pan.

Step 2

First make the base. Blitz the Oreos in a food processor until they're very finely crumbed and powdery. Pour in the melted butter and blend again till the mixture resembles wet sand and sticks together when pressed between your fingers.

Step 3

Transfer to the prepared loaf pan and press down firmly and evenly with the base of a measuring cup. Refrigerate for 30 minutes until firm.

Step 4

In the meantime, melt the white chocolate in a medium sized microwave-safe bowl in two 10 second increments on high, stirring in between each. Do not overheat the chocolate and when it's almost fully melted, stir gently to smoothen out any lumps. Set both aside to cool slightly.

Step 5

Im a stand mixer with a whisk attachment (or use a hand mixer and mixing bowl), beat 1/2 cup whipping cream on high speed for 3 to 4 minutes until it thickens to stiff peaks with very slightly rounded tops. See photos below.

Step 6

Add a spoon of the whipped cream to the melted white chocolate and stir gently to help loosen it. Fold in the rest of the cream gradually, working gently but quickly. It will at first look a bit uneven but will smoothen out in a fluffy and airy texture. Spread this mousse over the chilled Oreo base and refrigerate again for 30 minutes or until set enough that if you touch it, it no longer feels soft or sticks to your fingers.

Step 7

Next, melt the dark chocolate the same way as the white (use a separate, clean bowl to make sure nothing interferes with the melting of the chocolate) and set aside to cool slightly.

Step 8

Whip the remaining 1/2 cup of cream and fold it into the dark chocolate the same way to make another layer of mousse. Spread this over the layer of chilled white chocolate mousse and refrigerate another 30 minutes to 1 hour until set.

Step 9

Use the paper overhang to lift the mousse cake out and slice into 6 wedges or smaller rectangular slices if you prefer. Sprinkle with sea salt just before serving.

Step 10

Store in an airtight box for 4 to 5 days in the fridge. Let individual slices sit at room temperature for 15 to 20 minutes to soften slightly before eating. You can also freeze this for longer. Happy not-baking!