Complete the baking judge's polite but honest feedback on a disastrous dessert.

"I must admit, this cake is _____ a brick, but it certainly has a unique charm!"

The correct answer is as hard as.

When we use "as...as" to show that two things are equal in some way, we always use the base form of the adjective (hard), not the comparative form (harder). We also must use "as" on both sides, never "than".