Preheat the oven to 190°C, fan 170°C, gas 5. Line a 12-hole muffin tin with paper cases.
Put the flour, sugar, cocoa and baking powder into a large mixing bowl and stir to combine.
In a separate bowl, mix together the egg, egg yolk, milk, oil and vanilla extract. Add the wet mixture to the dry ingredients and stir until just combined, without beating. Add the chocolate chips.
Spoon the mixture into the paper cases, making sure each case is quite full. Drizzle the last of the caramel on the top of the muffins. Bake in the oven for 20 minutes until springy to the touch.