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.