Put eggs, salt, sugar, and baking powder into a large bowl. Whisk or beat well with an electric mixer. Add sieved flour, beat until combined.
Peel the apples, remove seeds and slice.
Add the apples to the bowl, stir well with a wooden spoon.
Butter the slow cooker pot or use the cooking spray. Pour the mixture into the slow cooker and turn it on BAKE. Bake for 60 to 80 minutes (test for readiness with a toothpick – it must be completely dry when it comes out).
Take the cake out of the slow cooker, put on a platter, sprinkle with icing sugar and serve.