Berkshire Hathaway has the most closely followed stock portfolio in the world, and for a few good reasons. For one thing, it's a massive collection of investments. Even after the recent stock market downturn, Berkshire's portfolio is worth about $329 billion, making up more than half of the conglomerate's entire market cap. Second, the portfolio has a long history of market-beating investments that many investors would otherwise overlook or consider "boring." And last but certainly not least, many of the investments in the portfolio were hand-selected by legendary investor Warren Buffett himself.

However, a few years ago, Berkshire reported an interesting move in its portfolio. The company added shares of two exchange-traded funds, or Pacer Swan SOS Fund of Funds ETF|ETFs. And while these are relatively small investments for Berkshire, it represents Berkshire's first major index fund investments. Here's a look at Berkshire's two ETFs and why they could be some of Warren Buffett's favorite investments even though they currently make up a tiny fraction of Berkshire's overall portfolio.