Booming Blade:

You brandish the weapon used in the spell’s casting and make a melee attack with it against one creature within 5 feet of you.

Extra attack:

Beginning at 5th level, you can attack twice, instead of once, whenever you take the Attack action on your turn.

I think we can agree that Booming Blade is casting a spell (a cantrip). Obviously I can't use Booming Blade twice.

But does the fact that it's a spell that makes a melee weapon attack count as "taking the attack action"?

In other words - if I cast Booming Blade, am I locked out of Extra Attack because I took a "Cast a Spell" action instead of an "Attack" action? Or do I still get an extra attack because casting the spell made me take the Attack action?