Good advice was already given here.
I'm with McMullan:
Don't use Erlenmeyer flasks
And most definitely not for
boiling starter wort!
But I can see the Erlenmeyer's flat bottom having charm on a stir plate. Many Mason jars today are quite flat bottomed too. Just test it out with water.
I use 1/2 (U.S.) gallon glass pickle jars, to make 1.6 liter starters. That leaves just enough headspace.
Use a kitchen pot
Use a kitchen pot to boil your starter wort. Stainless is preferred, but anything will work as long as you can clean the inside well. And it needs a well fitting lid.
Heat, mix, then boil your DME, or saved-out wort from a previous batch, in the pot.
Chilling
When done boiling, turn heat off, then cover with the sanitized lid. I'd sanitize the pot's rim too before putting the lid on. Let it steam up inside for a few minutes, then put the (now covered) pot in a sink or tub with cold water. No need for ice here, water works fine.
To speed things up, give the chilling water a stir every few (5) minutes or so.
Most importantly: don't open the pot!
When the water is getting too warm, drain it, and use fresh cold water. Or move the pot to a 2nd tub with cold water. You could add some ice to the chilling water now, if you're in a hurry, but it's really not necessary. It will get to room temps quite fast without ice.
You can do other things while it's chilling, like prepping your work area, cleaning and sanitizing jar(s), etc.
Important note in regard to risk of botulism:
You can keep saved-out, leftover wort from brew day in the fridge for a few days. But if you need to store it longer, freeze it!*
I use plastic cottage cheese or sour cream tubs. The 48 oz size is the most useful, but anything will work. Keeping the saved-out wort at a higher gravity is advantageous for storing, takes up less volume. Dilute to proper starter wort gravity when preparing the starter.
* Most important reason for freezing is a very small but realistic chance of botulism growing in wort, even when chilled.