Prepare a large rimmed baking sheet or jelly roll pan by greasing it with some butter and lining it with parchment paper. It also helps to grease on top of the parchment paper so the bars release easily.
Mash the bananas in a large bowl. It's easy enough to use a fork for this job, but you can also use a potato masher if you'd rather do that.
Add the sugar, Greek yogurt, eggs, oil and vanilla to the bowl with the bananas and mix these ingredients together with a wooden spoon until they're well combined.
In a separate bowl, combine the flour, baking soda and salt with a wire whisk. Dump the dry ingredients into the wet ingredients and mix just until everything is combined and you can't see any streaks of flour.
Pour the batter into the prepared pan and add it to the oven. Let it bake at 375 degrees Fahrenheit for about 22-25 minutes, or until the top is a nice golden brown and it passes the toothpick test (insert a toothpick into the centre and if it comes out clean, it's done!).
Let the banana bread cool in the pan until it is cool completely. It helps to place the pan on a wire rack while it's cooling.
While you're waiting for the banana bread to cool, you can make the frosting. Add the butter to the bowl of your stand mixer fitted with the paddle attachment. Whip the butter on high speed for about 2 minutes, scraping down the sides of the bowl every 30 seconds or so to make sure the butter is whipped evenly.
While the butter is whipping, it's a good idea to boil some water in your kettle. You don't need much, only 2 tbsp, but you must add it right after it boils so it's a good idea to get it started now.
Add the icing sugar and the milk at the same time and combine these ingredients with the butter, first on low speed and then on high speed until everything is combined well and the mixture is completely smooth (about 3 minutes). Be sure to scrape down the sides of the bowl every 30 seconds or so to make sure all the ingredients are incorporated.
Once the kettle has boiled, add the boiling water very slowly as the mixer is on low speed. Once all the water has been added, turn the mixer to high speed and let the frosting whip for about 3 more minutes or until the frosting becomes more pale in colour and achieves an extremely smooth consistency.
Spread the frosting over the banana bread using an offset spatula. Dust with cinnamon (optional), cut into squares and serve!
These banana bread bars keep in an airtight container for up to 3 days, but they are best enjoyed the day they're made.