Lightly grease a 7 inch springform pan with either oil or cooking spray.
In a medium bowl, whisk together eggs, milk and butter.
In a large bowl, combine flour, baking powder, baking soda, sugar and salt.
Pour wet ingredients from the medium bowl into the large bowl with dry ingredients and use a fork to cream together mixture until combined. Do not overmix, as some lumps are fine.
Transfer mixture to greased springform pan, then swirl the jam over the top of the cake mix and sprinkle on the pistachios.
Add the 250ml water to the Instant Pot and insert steam rack/trivet and place the cake pan on top.
Secure the lid and select Pressure Cook (or Bale) and adjust the time to 30 minutes. When timer beeps, allow the pressure to release naturally for 5 minutes, then quick-release any remaining pressure until float valve drops and then unlock lid.
Remove the pan from the inner pot and leave to cool for 30minutes.
Top with the chopped walnuts and a sprinkle of icing sugar, slice and serve.