Use a clean towel or dry pastry brush to dust off each Pop-Tart, removing any stray crumbs.
Combine all of the icing ingredients in a bowl and mix until a smooth paste forms. Put some of the icing into a piping bag fitted with a small, plain tip. Pipe a boarder of icing around the perimeter of each tart, leaving about a 1/4” margin.
Thin the remaining icing, a tablespoon of water at a time, until it reaches a pourable consistency. Pour a tablespoon of icing onto each tart and use the tip of a metal spatula to help it reach all of the corners. Pop-Tarts only have a thin smear of icing on top, so easy does it.
Let the icing dry for about 3 minutes or so before adding the sprinkles. Homemade or store bought, the sprinkles tend to bleed if they go on too soon.
The Pop-Tarts now need to dry, excruciatingly, overnight. They taste waaaaay too fresh the first day, and the icing will still be sort of damp and moist inside, which isantithetical to the Pop-Tart experience. After 12 hours of air-time, you’ll have unbelievable Pop-Tart perfection.
I know curiosity will get the better of you and you’ll try a freshly iced Pop-Tart even though I warned you. But then, a few days later, you’ll have another one, and you’ll say to yourself, “Holy Crap. That kid from BraveTart sure knows her homemade Pop-Tarts, this is incredible!”
And, on that note: These homemade Pop-Tarts last forever. I’d say they last at least two weeks. They become tastier over time. It’s a mysterious process I’ve never witnessed in any pastry before, but like real Pop-Tarts, aging on the shelf a few weeks really ups their game. (The gluten free version is especially friendly to aging. Even more delicious after a few days!)
You can re-warm these homemade Pop-Tarts in a toaster, but for only 10 seconds or so on the lowest setting. Too long in the toaster renders them strangely limp. Consider yourself warned.