Preheat the oven to 350°F (175°C). Grease and flour a cake pan.

In a mixing bowl, combine flour, sugar, cocoa powder, baking powder, baking soda, and salt.

Add egg, milk, oil, and vanilla extract. Mix until smooth.

Stir in boiling water until well combined. Pour the batter into the prepared cake pan.

Bake for 30 minutes or until a toothpick inserted comes out clean. Let cool.

In a mixing bowl, beat the softened butter until creamy.

Gradually add powdered sugar, heavy cream, and vanilla extract. Beat until fluffy.

Fold in the crushed Oreo cookies.

Once the cake is cool, spread the frosting evenly on top and sides.