Pulse the Nilla cookies and walnuts in a food processor until crumbled, about 30-40 seconds. Pour in the melted butter and pulse for a further 10-20 seconds until the mixture comes together. Press the cookie mixture to cover the bottom of the pan. Refrigerate 30 minutes to firm up.
In a microwave safe bowl add the dark chocolate and cream and heat at 30 second pulses until melted. Stir together until the mixture is combined. Let the mixture sit for 5 minutes to cool. Once the mixture has cooled, but not hardened, add in the chopped malt balls and stir to combine.
Take the cookie base out of the fridge and pour the truffle filling on top, gently spreading the mixture evenly over the cookie base. Place back into the fridge for a further 20-30 minutes for the truffle filling to set.
In a small saucepan add the condensed milk, cream, corn syrup, vanilla and sea salt. Bring the mixture to a boil, stirring constantly. Lower to a simmer and let it gently bubble for 15-20 minutes (a little longer if you like a darker caramel), stirring often so it doesn't burn. Set it aside to cool for 5 minutes before pouring over the dark chocolate truffle layer. Gently smooth the caramel with a spatula over the fudge layer and place back into the fridge for another 15 minutes to cool and firm up.
Remove from the refrigerator and pour over the melted milk chocolate over the top of the salted caramel. Quickly spread the milk chocolate evenly over the top of the caramel with a spatula. Quickly press the malt balls into the milk chocolate leaving a little sticking out of the top as garnish for each square. This should make 16 squares. Place back into the fridge and let it chill and firm up for 1 to 2 hours before cutting.