This Warren Buffett-Approved Investment Could Turn $300 a Month Into $1 Million

Warren Buffett is known for his stock-picking strengths. After all, they helped him lead Berkshire Hathaway to market-beating returns decade after decade. And Buffett, committed to long-term investing, has held some of these winning companies, from Coca-Cola to American Express, for many years. He once said that Berkshire Hathaway's ideal holding period was forever, and he's certainly followed through on that.

But Buffett's stock portfolio over time didn't include only stocks. It also held a particular asset that he recommends for pretty much any investor. Over time and without fail, this investment has delivered growth. This means that if you bought it and held on for a number of years, you would have seen your money grow. In fact, this particular asset could even turn a $300 monthly investment into $1 billion over time.

Let's check out this Buffett-approved investment that should be on everyone's buy list (and find out exactly how it could supercharge your portfolio.)

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Warren Buffett's investing advice

First, a quick note about Warren Buffett. The billionaire guided Berkshire Hathaway's investing decisions for 60 years, but at the start of this year handed that job over to Greg Abel. Buffett isn't absent from the company, however. He remains chairman and still readily shares his thoughts on investing and the market, both within the office and in public. As mentioned, Buffett built a successful career thanks to his ability to recognize a strong company, invest in it at the right price, and hold on for the long term.

Stock picking is a crucial part of investing, and Buffett has offered us a great deal of guidance and examples. But, in addition to building a portfolio of individual stocks, Buffett suggests one particular investment that's the right move for any investor. This is an asset that allows you to bet on something Buffett strongly believes in: the strength of the American company.

"American business has done wonderfully over time and will continue to do so (though, most assuredly, in unpredictable fits and starts)," Buffett wrote in his 2013 letter to shareholders.

The best way to gain access to the companies driving the U.S. economy? By investing in a fund that tracks the S&P 500. The famous benchmark rebalances regularly to ensure it includes the key companies of the times, and funds that track it must mimic any changes. This means that, by investing in one of these funds, you will always have exposure to the most significant companies of the day.

Buffett particularly likes the Vanguard S&P 500 exchange-traded fund (NYSEMKT:VOO). He's held shares of it in the past and has said that in the case of his death, he's advised trustees to put most of his cash in such a fund to benefit his wife.

Returns you can count on

Now, let's consider how investment in the Vanguard S&P 500 could build wealth for you. The S&P 500 has delivered an annual average return of 10% over time. So let's assume this continues. If you invest $3,000 initially in a fund that tracks it -- such as the Vanguard fund -- and then make a monthly contribution of $300 over 35 years, the value of your investment could top $1.2 million. With the magic of compounding at work, you could score an enormous win by consistently contributing to this S&P 500 investment.

If your investment horizon is shorter than 35 years, don't worry -- you still can benefit significantly by buying shares of an S&P 500 index fund and contributing to it on a monthly basis. The index has experienced down years here and there, but over time, it's always progressed and rewarded those who have stuck with it. This is because it is composed of quality companies that generally advance over the long run.

And because you are investing for a number of years and relying on compounding, you can add to this position throughout any market environment, from a bull market to a bear market. All of this means if you're looking for a generally safe investment and one that might even be worth millions, this Warren Buffett favorite makes a fantastic buy any time.

American Express is an advertising partner of Motley Fool Money. Adria Cimino has positions in American Express. The Motley Fool has positions in and recommends American Express, Berkshire Hathaway, and Vanguard S&P 500 ETF. The Motley Fool has a disclosure policy.