Crush bananas with lemon juice in a bowl and set aside.

Prepare a boiling water canner.

Heat glass jars, lids, and bands in simmering water.

Combine mangoes, bananas, and split vanilla bean in a large saucepan.

Stir in fruit pectin.

Add butter to reduce foaming, if desired.

Bring the mixture to a full rolling boil over high heat, stirring constantly.

Add sugar and banana liqueur (if using), stirring to dissolve.

Return mixture to a full rolling boil.

Boil hard for 1 minute, stirring constantly.

Remove from heat and skim off any foam.

Carefully remove the vanilla bean.

Ladle hot jam into hot jars, leaving 1/4 inch headspace.

Wipe the jar rim and center the lid on the jar.

Apply the band until fingertip tight.

Process in a boiling water canner for 10 minutes (adjust for altitude).

Remove jars and cool.

Check lids for seal after 24 hours.

Label and store.