Pop-Tarts or the Toaster Pastry
So the Pop-Tarts are flat rectangular toaster pastries & "Toaster Pastries" are a type of pastry designed to be "safely heated in a toaster".
The origin of Pop-Tarts were the "Country Squares" created in 1963 & adapted from a process of enclosing dog food by the company Post Cereals, but then since Post revealed the idea before it was ready, the principal competitor Kellogg developed quickly its own version & released it under the name of Pop-Tarts before Post product, it then had much more success then the "Country Squares" probably because of the name of this last, the term "Country" was associated then with being backward and boring.
Pop-Tarts are Kellogg's most popular brand to date, with more than 2 billion Pop-Tarts sold each year. They are distributed mainly in the United States, but also in the United Kingdom and Canada. Pop-Tarts were discontinued in Australia in 2005. To some extent, the brand name "Pop-Tart" has become a genericized trademark describing any toaster pastry.
In 1971 an advertisement campaign for Pop-Tarts was launched featuring Milton the Toaster a cartoon character, the campaign was terminated few years later when a commercial showed a child hugging the toaster, leading to complaints that children might imitate the commercial and burn themselves, I found a video of a TV commercial with Milton the Toaster from 1975, enjoy these 30 seconds :D
Pop-Tarts & Milton the Toaster (Sorry couldn't embed it)