Cut the butter into cubes and leave at room temperature to soften.
Toast and roughly chop the walnuts.
Pre heat the oven to 175ºC.
Fit the K beater to the machine.
Grease the large baking tray.
Add Ingredients 1 (egg yolks, sugar, butter) into the Mixer bowl.
Attach the Mixer bowl to the machine and fit the splash guard.
Mix on speed 4 for about 2 minutes or until combined.
Add Ingredients 2 (sour cream, vanilla extract) into the Mixer bowl.
Add Ingredients 3 (bicarbonate of soda, vinegar) into the small bowl.
Mix until combined.
Transfer the contents of the small bowl into the Mixer bowl.
Mix on speed 4 for about 1 minute or until combined.
Add Ingredients 4 (baking powder, flour) into the Mixer bowl.
Mix on speed 3 for 2 minutes or until combined.
Transfer the dough to a worksurface.
Wrap with plastic wrap.
Chill in the fridge for 30 minutes.
Transfer the dough to a lightly floured worksurface.
Roll out the dough into a rectangle, roughly to 33 cm x 23 cm (13” x 9”).
Transfer the dough to the large baking tray.
Spread the top of the dough with the first item of Ingredients 5 (raspberry jam).
Sprinkle the top of the dough with the last item of Ingredients 5 (walnuts).
Retrieve, clean the Mixer bowl and fit the Whisk tool.
Add the first item of Ingredients 6 (egg whites) into the Mixer bowl.
Whisk on speed 6 for 1 minute or until soft peaks form.
With the machine running, gradually add the last 2 items of Ingredients 6 (caster sugar, salt) into the Mixer bowl.
Whisk on speed 6 for 2 minutes or until stiff peaks form.
Spoon the meringue onto the dough and spread evenly.
Bake for 40 minutes at 175ºC.
Leave to cool.
Slice into squares.
Serve.