Learn Swedish: Vilken förstående chef! - What an understanding boss!

Förstående (understanding) is a present participle functioning as an adjective here. In Swedish, present participles end in either -ande or -ende, and they are invariable. This means their spelling never changes, regardless of whether the noun is an en-word, an ett-word, or plural!

How do we know to use -ende instead of -ande to turn the verb förstå into förstående?

For most verbs, you add -ande to the verb stem (like spännande). However, if the verb ends in a stressed long vowel, you add -ende instead. Since förstå ends in the stressed vowel å, it becomes förstående. You will also see this rule in words like boende (living) and troende (believing).

I learned that vilken means "which". Why is it translated as "what an" here?

You are right that vilken is usually the question word for "which". However, when used in exclamations like this one, vilken (as well as vilket and vilka) translates to "What a...!" Because chef is an en-word, we use vilken.

What would this sentence look like if we were talking about multiple bosses ("What understanding bosses!")?