Line 2 muffin trays with 12 muffin liners, placing one liner in every other cavity. Preheat the oven to 200°C fan.
Heat the milk and butter together (in the microwave or in a saucepan on the stove) until the butter has melted and the mixture is hot. Whisk in the instant coffee granules until dissolved, then add the cocoa powder and whisk until smooth. Set aside to cool to room temperature.
In a large bowl, whisk together the flour, sugar, bicarbonate of soda and salt. Add 2/3 of the chocolate chunks and mix until the chunks are nicely coated in the dry ingredients.
Add the eggs, sour cream, sunflower oil and vanilla extract to the cooled milk mixture and whisk until combined.
Pour the wet ingredients into the dry ingredients and mix briefly with a spatula until just combined.
Divide the batter evenly between the 12 muffin cases (I filled each case with 2 regular-sized ice cream scoop's worth of batter). Sprinkle the muffins with the remaining chocolate chunks, then place into the preheated oven and bake for 5 minutes. After 5 minutes, turn the temperature down to 150°C fan and bake for a further 15 minutes, until a skewer comes out clean, or with just a few moist crumbs. Bake the two trays separately, one after the other, for best results.
Immediately after the muffins have baked, remove them from the tin and transfer them to a wire rack to cool completely before filling.