Preheat the oven to 350°F. Grease a 9x13-inch baking pan.
In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, baking powder and salt.
Add the buttermilk, oil, vinegar and vanilla and whisk until thick and smooth. Whisk in the hot coffee until the batter is glossy, then pour it into the prepared pan.
Bake for 30-35 minutes, until a toothpick inserted in the center comes out clean. Let the cake cool for 10 minutes.
Using the round handle of a wooden spoon, poke holes all over the warm cake about 1 inch apart, pushing almost to the bottom.
Whisk the condensed milk and warm caramel sauce until smooth, then pour it slowly over the cake, letting it sink into the holes. Cool to room temperature, then refrigerate for at least 2 hours.
Whip the heavy cream, powdered sugar and vanilla to stiff peaks and spread it over the chilled cake. Drizzle with the caramel sauce, sprinkle with toffee bits and keep chilled until serving.