Crush biscuits: Break the butter biscuits/cookies into smaller pieces and process them in a food processor until they resemble breadcrumbs. Alternatively, place the biscuits into a freezer bag and crush them finely with a large rolling pin.
Crust: Melt the butter in a small saucepan, add the butter to the crushed biscuits and mix. Press the mixture into a buttered springform of about 24-26 cm/ 9.5-10 inches diameter. Refrigerate while you make the filling.
Soak gelatin: Cover the gelatin with cold water and soak for 10 minutes or according to the packet's instructions.
Dissolve gelatin: Place the sugar and the milk into a small saucepan. Heat while stirring until the sugar is dissolved. Bring to a boil and stir until the mixture thickens slightly. Remove from the heat and let cool slightly. Squeeze the bloomed gelatin barely and mix it thoroughly into the sugar mixture.
Combine: Place the cream cheese and yogurt into a bowl. Mix well and add the caramelized sugar. Place the filling into the fridge until it begins to set. It should only begin to set, not set entirely.
Whip: In the meantime, beat the heavy cream until stiff. Fold it carefully into the cream cheese mixture.
Fill: Halve 3 bananas lengthwise. Place the banana pieces onto the pie crust. Carefully pour the cream cheese filling over the bananas.
Refrigerate the cake for at least 3-4 hours or overnight.
In the meantime make the candied walnuts and leave them cool completely on a piece of baking paper until ready to use.
Caramel: Heat a non-stick pan over medium heat. Add the sugar and the butter. When the sugar and the butter start to melt, add the chopped walnuts and stir well to coat them with the butter-sugar mixture all over. Keep going for about 3-4 minutes or until the walnuts are nicely caramelized.
Caramelize walnuts: Place the walnuts onto a piece of baking paper. Separate slightly so that the walnuts don't stick together into a clump but instead lay in a single layer or as separate as possible. Let cool completely.
Only place the candied walnuts and the last banana on top of the pie just before serving.
Slice the remaining banana and turn the slices into lemon juice. Arrange the banana slices around the edges of the pie.
Chop the candied walnuts and place them on the cake inside the circle you formed with the banana slices. Decorate the cake with some lemon zest as well, if desired.