My understanding is that senior government officials usually exercise powers by recording their decision to do so, such as by sending an email or signing a piece of paper. For example, the US President might sign a printed copy of an Act or a proposed executive order. That Act or order would usually be subsequently published, and in some cases the law requires the decision to be publicised in a particular way after it is made (e.g. published in the Federal Register). However I am not sure if there are any legal restrictions on the procedure used to initially record the actual making of the decision.
On 23 August 2019, the US President tweeted that 'American companies are hereby ordered' to do something, and retweeted the tweet on the official 'POTUS' Twitter account. It's not clear to me whether this tweet was intended to constitute a formal order or was referring to a separate document which would contain the formal order.
In any case, this made me wonder (as a general proposition) whether it would be possible to dispense with any other more formal document, and just issue the order via Twitter. For example, could the President pardon someone by tweeting 'XYZ is hereby pardoned for all her crimes against the United States'?
Can the US President exercise their powers by tweeting?