Export an existing post design to a post template

Hi there!
Happy new year to all "stacks"

I would like to know how to build a post templates as a predefined templates from an existing post format.
- we created a post, which we would like to use as an template for our following posts
Is there something like "export as template" or export as a file, and then import ?

Or any tip or hint how ?

Regards
Klaus