For caramel, boil brown sugar, golden syrup, 1/4 cup heavy cream, and 1/4 tsp. salt in large pan over medium heat til mixture reaches 240 on a thermometer, 2 to 5 minutes. Whisk in remaining 1/2 cup heavy cream and transfer to a glass measuring cup with a pour spout; let cool to room temperature.
Preheat oven to3 50. coat a 9 x 13 inch baking dish with cooking spray.
Whisk together flour, sugar, cocoa powder, baking soda and 1 tsp. salt. Mix hot water, oil, vinegar, vanilla and espresso powder; whisk into dry mixture. Pour batter into prepared baking dish and bake til toothpick inserted in center comes out clean, about 25 minutes. Poke cake all over with fork while still hot.
Spread sweetened condensed milk evenly over cake and allow to soak in. Spread caramel evenly over cake. Allow cake to cool to room temperature, then cover and chill til cold, at least 2 hours or overnight
For whipped cream, beat 3 cups heavy cream to nearly stiff peaks. Add powdered sugar and beat to mix. Spread whipped cream over chilled cake, then sprinkle toffee bar over the top.