In a bowl, beat 3/4 cup sugar with butter until light and fluffy. Beat in egg yolks until incorporated. Stir in lemon zest and juice. Add salt, then flour and mix until blended. Gradually add milk, beating to make a smooth batter.
In a separate bowl, with clean beaters, beat egg whites until soft peaks form. Add remaining 1/4 cup sugar and beat until stiff peaks form. Fold into lemon mixture, then fold in raspberries.
Pour mixture into a 6-cup lightly greased baking or souffle dish. Cover with foil and tie tightly with a string. Place dish in a large oval slow cooker and add boiling water to come 1 inch up the sides. Cover and cook on HIGH for 3 hours, until the cake springs back when touched lightly in the center. Dust lightly with confectioner's sugar and serve.
NOTE: Blueberries may be substituted for the raspberries.