Cocoa butter crystals in chocolate frosting

We have been making a chocolate frosting recipe for 15+ years. At it's best it is creamy, smooth, fudgy, and stays a beautiful, spreadable consistency at room temperature. We make it in a batch that can frost three layer cakes, so we can have it on the shelf ready to go at all times for last minute cake orders.

At times, though, we have trouble with it "crystallizing". It gets little grains of cocoa butter in it, sometimes just a few, and sometimes so many that it just looks terrible and loses it's spreadability.

I'm pretty certain the grains are cocoa butter because they melt completely at body temp if you rub them between your fingers. The frosting tastes fine and because the crystals melt so easily, you don't notice them in the mouth. If we rewarm the frosting, it smooths out, but then either the crystals reform, or the whole batch of frosting sets up way too hard, like a very firm ganache, and it can't be spread on cakes. Here is the recipe:

• 3.5# Unsalted butter, softened to warm room temp, until shiny. (We use only Plugra as our regular butter from our foodservice supplier varies way too much in water content.)
• 4 oz. Cocoa Powder (we use Bensdorf Dutched now, but have used various brands with no consistent negative or positive effect)
• 8 fl oz. Hot water - from our coffee makers
• 3#10 oz Chocolate chips, melted (We use C'est Vivant brand from Bakemark- standard semisweet chip)-
• 6 fl oz Corn Syrup

Method: Cocoa powder is dissolved in hot water, cooled slightly and blended in to very soft butter. Chocolate chips are melted and blended in and then corn syrup blended in. Minimal blending is used as excessive blending makes the frosting light brown instead of dark and fudgy. (It is easily blended by hand, but we frequently use a 20qt mixer with a paddle attachment.)

I know the recipe can work, because we have made this frosting for so long, and have gone for months at a a time without the crystallizing problem, then it crops up and we try to figure it out, then it stops for a while, etc. We have tried different chocolates, but honestly, the better chocolates don't necessarily make a better frosting - they tend to make it too soft or too hard, or it tastes weird, and they are expensive, or we have to pay for shipping, or we can't even get it shipped in the summer, or we have to buy a 50# case. It's really best for us to just use the chocolate chips we have in inventory for so many other uses. Other things we have tested and eliminated as contributing factors - meaning crystallization has happened under both circumstances: brand of corn syrup (generic foodservice product vs Karo), eliminating any water droplets being introduced after the frosting is finished, melting the chocolate chips in a microwave vs double boiler, hand mixing vs machine, and probably some other things I can't think of right now. The thing that seems to make the most difference is the temperature the frosting is held at as it sets up after mixing. Too cold an environment seems to exacerbate the crystallizing. Too warm an environment and the frosting stays very soft - almost a liquid and too soft to frost anything but a frozen cake. So we store it in various places depending on the season.

In the summer when the main kitchen gets too hot, it goes in the storeroom, in the winter when the main kitchen stays below 68F all day, it goes on a server shelf in the dining room that stays warmer or on top of the ice machine. So yes, the problem is definitely worse in the winter. I really love this frosting when it is right - it has the right texture, a good, American style chocolate flavor and a beautiful spreadability that makes our humble diner style chocolate cake look like something from a photo shoot.

I'm wondering if anyone has suggestions for making this recipe more foolproof. Is there something I can add or a technique I can try to prohibit or delay or minimize the crystallizing of the cocoa butter? Would an emulsifier help? (If I could find the perfect storage temp, that might work, but I don't really have a spot for storage that is consistently the optimal temp in our building over the course of all the seasons here in Michigan, USA.)