For the Cake:

1. Preheat oven to 350 degrees. In a stand mixer fitted with a paddle attachment, add flour, sugar, cocoa powder, baking powder, baking soda, and salt. Mix on low speed until blended together.
2. Add milk, eggs, and sour cream and mix on medium low just until ingredients come together. 
3. Add boiling water and mix briefly.
4. Pour batter into a 9 x 13 baking pan that has been coated with butter and flour. Bake for 25-35 minutes. Test cake by poking cake with a toothpick in the middle. If the toothpick comes out clean, it is does. If the toothpick is gooey, add a few more minutes and check again.
5. Allow cake to cool completely and poke small holes all over the cake.
6. Pour sweetened condensed milk and 3/4 of the caramel into a small pot and warm until sauce is thin. Do not simmer or boil, but allow the sauce to warm up.
7. Pour sauce over cake and smooth over with a spoon so the sauce goes into all of the little holes. 
8. Generously sprinkle toffee bits on the cake.
9. Whip the heavy cream and sugar until stiff peaks form. Spread evenly over the cake.
10. Sprinkle with more toffee bits and drizzle remaining caramel over the cake. If caramel is too firm, warm in microwave for 10-20 seconds to soften it. It should drizzle easily over the cake.
11. Eat immediately or refrigerate until ready to eat.