Peel, core and thickly slice apples. Toss with lemon juice in a bowl, then stir in cinnamon and 3/4 cup sugar. In a medium bowl, combine the remaining 1/4 cup of sugar, salt, baking powder and flour. Add the butter, milk and vanilla to the flour mixture to form a dough. Spread the dough over the filling, cover and cook on high for 2-2 1/2 hours or until a toothpick inserted into the dough comes out clean. Slow cookers vary; cooking times are approximate.