Chicken Filling: Place the chicken breasts into a large saucepan. Pour in the water and season with chilli powder, salt, cumin, black pepper, garlic granules, onion powder and cayenne pepper. Bring to a simmer over high heat, then reduce heat to medium-low and simmer 15 minutes.
After 15 minutes, stir in green chillies, onion and garlic then continue simmering until the liquid has reduced to 250ml. Reserve remaining chopped green chillies. Remove the chicken, shred with two forks and return to the onion mixture.
Sour Cream Sauce: melt the butter in a small saucepan over medium heat. Stir in the flour and cook for 1 minute. Whisk in the water and stock cube until the water has thickened and the stock cube has dissolved, about 4 minutes. Whisk in the green chillies and the sour cream then season to taste with salt and pepper. Keep warm.
To Cook: Heat oil in a deep fat fryer or large saucepan to 190 degrees C.
Place a tortilla onto your work surface, then spoon about 5 tablespoons of the chicken filling halfway between the bottom edge and the centre of the tortilla. Flatten the filling into rectangle shape with the back of a spoon. Sprinkle some of the Cheddar cheese over the filling. Fold the bottom of the tortilla snugly over the filling, then fold in the left and right edges. Roll the chimichanga up to the top edge, forming a tight cylinder; secure the ends with cocktail sticks. Repeat with the remaining ingredients.
Cook the chimichangas, 2 at a time, in the hot oil until they are crisp and golden brown on both sides, about 1 minute per side. Drain on a kitchen towel lined plate and remove cocktail sticks. Top with sour cream sauce to serve.
Tinned chopped green chillies can be purchased online. Alternatively, you can use fresh chillies to taste.