For the base, blitz biscuits in a food processor, add butter, then pulse to combine. Line the base of a lightly greased 24cm springform tin with baking paper, then spread mixture evenly over the base and sides. Press firmly to even out the base and sides, which should be 5mm thick. Refrigerate for 30 minutes, until firm.
For the filling, slice bananas into 1.5cm discs, pack them closely together on a foil-lined oven tray, and sprinkle with 2/3 cup sugar and grill under high heat until the sugar caramelises into a golden colour (about 8-10 minutes). Allow to cool.
When banana has cooled, process with the remaining sugar and the lemon juice until just combined. Add 1/3 of the egg mixture and process until just combined, then add the cream cheese and process until just combined. Add remaining egg mixture and process until just combined. Add sour cream and process until smooth.
Preheat oven to 170°C. Place prepared tin onto a baking sheet. Pour filling into crumb crust, and tap tin gently to remove any bubbles.
Bake for about 45 minutes, until almost firm. It will be a little wobbly in the centre, but will firm as it cools.
For the topping, combine ingredients in a bowl and whisk until smooth.
Let cooked filling stand for five minutes at room temperature before pouring over topping. Use a spatula to spread it evenly. Return cheesecake to oven for five minutes.
Let cheesecake stand at room temperature for 10 minutes. Gently run a knife around the inside of the tin to release the crumb from sides of the tin. Refrigerate until cold and firm, preferably overnight.
Place 1 tbsp water and 50g castor sugar in a small saucepan, stir to combine. Bring to boil over high heat, swirling pan occasionally until toffee is rich golden brown. Pour onto baking paper-lined oven tray, set aside to cool, then chop finely.
Remove cake from pan, scatter with toffee, slice with a hot knife and serve.