Remove the green husks and all of the “silk” from the corn. Holding each cob upright on a chopping board, cut the kernels from the cob. (You should end up with about 1-1⁄2 cups loose corn kernels.) Set the kernels aside.
Heat the canola oil in a large saucepan, add the onion and fry over medium heat until softened, but not browned.
Add the potatoes and corn kernels to the pan and cook for another 5 minutes, stirring frequently.
Pour in the chicken stock and bring to a boil. Reduce the heat and simmer gently for 5 minutes or until the potatoes are just tender.
Stir in the milk and the chopped tarragon. Season with salt and pepper. Cook, stirring, for another 2–3 minutes.
Pour half of the mixture into a food processor or blender and blend to a coarse texture, not to a purée. Return the mixture to the pan. Add the chicken and stir to mix.
Set the soup over a low heat to warm through.
Meanwhile, cook the bacon until it starts to brown. Drain on paper towel, then finely chop.
Ladle the soup into bowls, scatter on some bacon and a sprinkling of tarragon leaves, and serve.