There's a cheap and easy trick to boost banana in Weissbier even more: just add some Dextrose to the grist. Weissbier yeasts convert pure Dextrose to banana esters.
Lallemand published an article on their experiment where they added Dextrose, 10 g per 1 L of wort, fermented it with Lalbrew Munich Classic at 20°C and registered a significant increase in banana esters. Here's about that:
Could adding Glucose give more Banana flavour to your Hefeweizen?
There's also the Hermann Verfahren mashing method aimed to get the same effect without adding Dextrose: you step-mash half of the grains to a complete convertion and then add in another half of the grist in cold water to rest the whole mash at 45°C to naturally produce more Dextrose from the Maltose already present in the mash, and then you proceed with further stepped rests or with decoctions.
UPD:
Today I'm brewing a batch using the Hermann Verfahren Method, will ferment it with Lalbrew Munich Classic. I've been never fully satisfied with the level of esters I was getting from this yeast, so will see if it's different this time and if it's worth the effort of the double, six-stepped mashing. Will share the experience when the beer is ready.
My mashing schedule is like following:
Mash in 50% of the Grist:
Mash 20' @45°C / 113°F (1:3 grain to water ratio)
Mash 45' @62°C / 144°F (1:5 grain to water ratio)
Mash 20' @70° C / 158°F
Add 50% of the Grist + cold water, whole mash is cooled to 45°C / 113°F
Mash 45' @45°C / 113°F
Mash 20' @62°C / 144°F
Mash 20' @70°C / 158°F
Boil 60'