Heat oven to 220C/200C fan/gas 7. Put all the dry ingredients together in a bowl and mix well with a whisk to distribute the cinnamon and baking powder. Add the wet ingredients and mix until just combined. Pour the batter into a piping bag and fill the doughnut pan until each hole is approximately three quarters full. Do this in batches if needed.
Bake for 9–10 minutes until risen, golden brown and the tops are springy to the touch. Allow to cool for a couple of mins then turn out onto a wire rack to cool completely. If the doughnuts have lost their holes during baking use a small cutter or piping nozzle to recut them.
Put your pink candy melts in a microwaveable bowl with 1 tbsp vegetable oil. Melt at 30 second intervals at a medium heat until silky and completely melted. Spoon the pink candy melt over the top of each doughnut wiping off any drips that fall down the edge. Leave on a wire rack until set (about 5-10 mins). Do not throw away the excess pink!
Meanwhile melt the green candy melts in the same way. Hold your doughnuts on the edge and roll them through the green candy melts only covering the outside not the pink. Leave to set.
Cut the chocolate chips in half to create watermelon seed shapes. Dip a cocktail stick into the pink candy melt and mark out the spots to place your seeds then stick on the chocolate chips.