Some of my biggest stock portfolio purchases in the past 12 months have been shares in an ASX passive income powerhouse — one that pays me a dividend every single month, no less.

At its closing price of $1.25 on Wednesday, this passive income stock pays a hefty (and fully franked) dividend yield of 5.28%. However, some of my purchases have given me a yield on cost as high as 6.35%.

This ASX passive income stock is none other than Plato Income Maximiser Ltd (ASX: PL8).

Plato Income Maximiser is a listed investment company (LIC). That means it is a company that invests in other assets. In this case, a Plato investment translates into buying a share in an underlying portfolio of other ASX shares.

Plato constructs this underlying portfolio with the goal of maximising franked dividend income for its investors. Some of its most recently reported largest holdings include the likes of Fortescue Ltd (ASX: FMG), Ampol Ltd (ASX: ALD), ANZ Group Holdings Ltd (ASX: ANZ), and Super Retail Group Ltd (ASX: SUL).

A 5.3% yielding passive income share?

Thanks to this portfolio of generous dividend payers, Plato is able to be a generous passive income payer itself. As we touched on earlier, this LIC pays its investors a dividend every single month, which is unusual on the ASX. Over the past 12 months, each of the 12 dividends investors have received has been worth 0.55 cents per share, equating to an annual total of 6.6 cents per share.

As mentioned, the current Plato share price of $1.25 gives the company a trailing dividend yield of 5.28%. However, my most recent purchase of Plato shares was fortunately at a price of $1.04. That means I was lucky enough to secure a yield-on-cost of 6.35%. That's a grossed-up yield of 9.07% with the value of those full franking credits included.

These kinds of yields are obviously appealing to passive income investors. Yet, it's not the only reason why I've been snapping up this stock over the past 12 months.

This company also happens to be a long-term market beater. As of 31 August, Plato shares have returned an average of 8.8% per annum (share price growth plus dividends and franking). Over the same period, an ASX 200 index fund has returned an average of 8.3% per annum.

So you've got a company that has proven to be a market-beater over the past three years that pays a 5.3% dividend yield right now. That's why I've been bulk-buying Plato Income Maximiser shares over the past 12 months and will probably continue to do so.