Preheat your oven to 350°F (175°C) and grease a round 9-inch cake pan.

In a mixing bowl, combine the flour, sugar, cocoa powder, baking powder, baking soda, and salt. Whisk until well-blended.

In another bowl, whisk the egg, milk, oil, and vanilla extract. Add this mixture to the dry ingredients and stir until just combined.

Carefully mix in the boiling water to create a thin batter.

Pour the batter into the prepared cake pan and bake for about 30-35 minutes, or until a toothpick inserted in the center comes out clean.

Allow the cake to cool in the pan for about 10 minutes, then transfer it to a wire rack to cool completely.

Once the cake has cooled, slice it in half horizontally to create two layers.

Spread a generous amount of softened chocolate ice cream on the bottom layer, then carefully place the top layer back on.

Wrap the cake in plastic wrap and place it in the freezer for at least 2-3 hours to firm up.