For the Pineapple Syrup, slice pineapple into 2cm thick slices. Reserve 2 of the slices for the Caramelised Pineapple. Dice remaining pineapple slices and pass through a slow juicer into a clean jug.
Measure out 1 cup of the pineapple juice and place into a medium frypan with 4 tablespoons of the caster sugar. Place over a high heat and bring to the boil, then lower heat to low and simmer gently until the liquid reduces and is light golden in colour. Adjust the tartness of the syrup with additional sugar if needed. Transfer to a small serving jug and set aside.
For the Caramelised Pineapple, remove and discard the pineapple core then dice pineapple into 3cm square pieces.
Place the butter into a medium frypan and place over medium heat. Once butter has melted, add pineapple pieces and cook, turning, until golden brown on all sides. Remove pan from the heat and set aside until needed.
For the Ricotta Doughnuts, heat oil in a medium saucepan or deep fryer to 170C.
To make the ricotta, place milk, cream, vinegar and salt into a medium saucepan and stir to combine. Place over a medium heat and bring to just below boiling point and mixture begins to just separate. Reduce heat to low and continue to simmer gently for 1-2 minutes. Remove from the heat and set aside for 5 minutes.
Line a fine sieve with muslin cloth and sit over a deep bowl. Pour the milk cream liquid carefully into the lined sieve and set aside to allow whey to drain for 10 minutes. Twist the top of the cloth and squeeze gently to ensure to remove the whey. Set the ricotta aside.
Place 225ml water, butter and 1 teaspoon of salt into medium saucepan over medium medium heat.
Once butter has melted, add flour and stir vigorously with a wooden spoon out until mixture pulls away from the edges of saucepan and forms a smooth ball.
Transfer dough to the bowl of a stand mixer fitted with a paddle attachment. Set aside to allow to cool for a few minutes. Once dough has cooled, add ricotta to the bowl of dough and mix on low speed until just combined, then add eggs, one at a time, and mix until the dough is soft and smooth, about 3-5 minutes.
Place caster sugar into a shallow tray and set aside.
Once oil has reached temperature, spoon heaped teaspoons of dough, in batches, into the oil and fry until golden brown all over, about 4-5 minutes. Remove doughnuts from oil, drain on paper towel then roll in the caster sugar until evenly coated. Repeat this process to cook remaining doughnuts.
To serve, divide Ricotta Doughnuts between serving plates and stack to one side of each plate. Add some pieces of Caramelised Pineapple to the other side of the plate. Re-warm the Pineapple Syrup and serve in the jug on the side.