Set your oven rack in the middle position. Preheat the oven to 350°F. Spray a 9x13x2-inch pyrex pan with nonstick spray.
PREPARE THE CAKE:
In a large bowl, sift together the flour, baking soda, salt and cinnamon.
Combine the sugar and butter in the bowl of a standing mixer with a paddle attachment (or use a hand mixer). Add the eggs, one at a time, beating well after each addition. Add the vanilla. Add the apples alternately with the sifted dry ingredients, beating after each addition. Fold in walnuts, if using.
Pour the cake batter into the prepared pan. Bake 1 hour, or until the cake pulls away from the sides of the pan and a tester inserted into the cake comes out clean. Cool on a rack 25 minutes, or until just warm.
MAKE THE TOFFEE TOPPING:
Melt the butter in a heavy metal saucepan over low heat. Add the brown sugar and salt and whisk until blended. Add the heavy cream, increase heat to medium, and stir with a wooden spoon until the mixture comes to a boil. Boil 5 minutes, stirring constantly. Remove from heat. The mixture will thicken as it rests.
Make several slits in the warm cake with a butter knife. Pour the sauce over the cake. Let the cake rest until the topping sets, about 15 minutes. Serve with vanilla ice cream or unsweetened whipped cream. Store loosely covered with wax paper at room temperature.
Notes
Nutritional information has been figured without using walnuts.