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?
… 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?
Yes. Extra Attack specifies that you must take the Attack action. With Booming Blade, you’re still taking the Cast a Spell action, the singular melee weapon attack is simply how the spell is cast, not an entire Attack action.
This is the same for all spells that require you to make a weapon attack as part of casting it. The only spells that kind of break this rule are the various “Smite” spells, but they’re worded like “When you next hit a creature with a melee weapon attack…”, and generally use a bonus action rather than an action, so you can still use your action for the Attack action.
Attack is a different action which would let you attack twice with Extra Attack.
You cannot Booming Blade your attacks with Extra Attack.
This is actually the topic of a Sage Advice:
Can you use green-flame blade and booming blade with Extra Attack, opportunity attacks, Sneak Attack, and other weapon attack options? Introduced in the Sword
Coast Adventurer’s Guide, the green-flame blade and booming blade spells pose a number of questions, because they
each do something unusual: require you to make a melee
attack with a weapon as part of the spell’s casting.
First, each of these spells involves a normal melee
weapon attack, not a spell attack, so you use whatever ability modifier you normally use with the weapon. (A spell tells
you if it includes a spell attack, and neither of these spells
do.) For example, if you use a longsword with green-flame
blade, you use your Strength modifier for the weapon’s attack and damage rolls.
Second, neither green-flame blade nor booming blade
works with Extra Attack or any other feature that requires
the Attack action. Like other spells, these cantrips require
the Cast a Spell action, not the Attack action, and they can’t
be used to make an opportunity attack, unless a special feature allows you to do so.
Third, these weapon attacks work with Sneak Attack if
they fulfill the normal requirements for that feature. For example, if you have the Sneak Attack feature and cast green-
flame blade with a finesse weapon, you can deal Sneak Attack damage to the target of the weapon attack if you have
advantage on the attack roll and hit.
Also, you actually can use Booming Blade twice in one turn because its a cantrip. The limit for spells cast only applies to leveled spells. The easiest way to Booming Blade twice is the Sorcerer metamagic Quicken Spell.
The limit for spells cast only applies to leveled spells.
This is not worded completely correct; you can cast two leveled spells in one turn (using Action Surge). You just cannot cast a leveled spell in the same turn as casting a spell using a bonus action.
As a caveat to what has been stated by others, if you're a bladesinger wizard, the Extra Attack there does allow for it to be used with Extra Attack since Bladesinger allows one of the attacks to be replaced with a cantrip cast.
And Eldritch Knight has War Magic, which does not quite do the same thing, but still makes casting Booming Blade almost always better than the Attack action from levels 7-11 (it’s not generally a BA heavy subclass, but occasionally you’ll want to use something like Second Wind.) Even after level 11, the bonus damage puts the DPR higher than 3-4 attacks, so the choice is between more damage upfront vs. movement deterrent with a higher damage punishment.
Others have already made the point that attacking is not the same as the attack action. The consolation is that the extra damage from Booming/Green-Flame Blade goes up at level 5 instead!