In a bowl, mix together the flour, cocoa powder, caster sugar and 75 g (3 oz) of the chocolate chips.
Combine the eggs, bananas, oil and yogurt in a jug or bowl, then pour the wet ingredients into the dry and mix to barely combine. Divide the mixture between the greased or paper case-lined cups of a 12-cup muffin tin and scatter over the remaining chocolate chips.
Bake in the preheated oven, 180˚C (350˚F), Gas Mark 4, for 18–22 minutes until risen and firm to the touch. Cool slightly on wire racks and serve warm, drizzled with warm toffee sauce.