An iron butterfly is a combination of a short straddle and iron condor. It's a great strategy to use during very high IV setups when you want to also reduce the capital required to hold the trade.
You'll build this strategy by selling both the ATM call and put strike (similar to a straddle) and then buying further OTM wings for protection (like an iron condor). The key is to use a wider spread in the strike prices to maximize the credit received.
For example, if a stock is trading at $50 you would sell the ATM call and put at $50 strike and then look to buy maybe the $45 or $40 puts below the market and the $55 or $60 calls above the market. The wider you can make the strategy the more credit received and higher probability of success.