Coarsely chop both the dark chocolate couverture and whole milk couverture using a large knife on a large cutting board.
Roughly chop the coconut fat into small pieces. Place the coconut fat and chopped chocolate in a heatproof bowl. Set the bowl over a pot with simmering water on low heat, stirring every 1–2 minutes until completely melted. Do not let water boil or splash into the bowl.
In another bowl, combine the eggs, powdered sugar, milk, and cocoa. Mix with an electric mixer for 2–3 minutes until smooth and well combined.
Pour the melted chocolate mixture into the egg mixture and stir until combined into a thick paste.
Rinse a loaf pan with cold water and line it with foil, or use a silicone loaf mold without foil.
Spread a thin layer of the chocolate mixture on the bottom of the prepared pan and smooth it out. Arrange Oreo biscuits in a single layer on top, placing them side by side or with small gaps depending on the pan width. Spread another layer of chocolate mixture over the Oreos. Repeat this layering until all Oreo biscuits are used. Finish with a final layer of chocolate mixture smoothed on top.
Cover the pan tightly with foil to prevent refrigerator odors and condensation. Refrigerate for at least 5 hours.
Remove the cake from the pan, slice it with a sharp knife, and serve. Optionally, decorate with whole or halved Oreo biscuits.