Preheat your oven to 350°F (175°C) and grease and line two 9-inch round cake pans with parchment paper.

In a large bowl, whisk together the flour, caster sugar, cocoa powder, baking powder, baking soda, and salt until well combined.

In another bowl, whisk the eggs, milk, vegetable oil, and vanilla extract until smooth.

Gradually add the dry ingredients into the wet mixture, stirring to combine.

Slowly whisk in the warm water. The batter will be thin, but this ensures a moist cake.

Divide the batter evenly between the prepared pans and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cakes to cool in the pans for 10 minutes before transferring them to a wire rack to cool completely.

In a large mixing bowl, beat the softened butter with an electric mixer until light and creamy, about 3-4 minutes.

Gradually add the powdered sugar, one cup at a time, beating well after each addition.

Add the vanilla extract and milk (if needed) to achieve a smooth, spreadable consistency.

Fold in the crushed Oreo cookies gently.

Place one cake layer on a serving plate and spread a generous amount of Oreo buttercream on top.

Place the second cake layer on top and frost the entire cake with the remaining buttercream.

Decorate the cake with whole Oreo cookies on top.

For best results, chill the cake in the fridge for about 30 minutes before slicing to allow the buttercream to set.

Slice and enjoy your delicious chocolate Oreo cake!