Preheat oven to 375 degrees.
In a medium mixing bowl, combine the flour, sugar, cocoa powder, baking soda, and salt.
In a small bowl, beat together the vanilla, egg, sour cream, milk, and oil.
Stir the wet ingredients into the dry until just combined.
Spoon batter into a greased donut pan.
To make the streusel, cut together the flour, sugar, and butter until you have small, pea-sized balls of butter.
Stir in the toffee bits.
Sprinkle the streusel over the tops of the donuts.
Bake for 8 minutes or until the tops spring back when you touch them.
Let the donuts cool in the pan until cool enough to handle.