Heat oven to 180C/160C fan/gas 4. Line a 12-hole muffin tin with paper cases. Sift the flour, cocoa, baking powder and cinnamon into a bowl. Stir in the sugar and cranberries. Microwave the chocolate on Medium for 1-1½ mins, stir, then set aside.
Mix the yogurt and milk with the oil and egg. Make a well in centre of the dry mix and gently stir in the liquid. Drizzle half the chocolate over the mix, gently fold in until swirled, then repeat with the remaining chocolate. Take care not to over-mix. Spoon the mix into the muffin cases and bake for 15-20 mins until risen and firm to the touch.