Weigh out 125 g / ¾ cup plain flour, 2 tsp baking powder, 25 g / 5 tsp cocoa powder, 90 g / ¼ cup + 3 tbsp caster sugar and 60 g / ⅓ cup milk chocolate chips and place them all in a mixing bowl.
Measure 125 ml / ½ cup of milk, 1 egg, 1 tsp of vanilla bean paste and 40 ml / 8 tsp vegetable oil in a separate jug and mix together.
Add the wet ingredients in the jug to the dry ingredients in the bowl a third at a time. Mix roughly until combined, don't overmix.
Using a pastry brush and a little extra vegetable oil, carefully oil each hole in the silicone mould.
Fill each hole in the mould with 2 to 2.5 teaspoons of the mixture, filling each hole half way. You will only use half of the mixture, save the rest for a second batch once the first have finished cooking.
Put the silicone mould in the slow cooker, put the lid on with a dish towel or some squares of kitchen towel under the lid to catch drips, and cook on high for 1 hour 45 minutes approx.
Important: slow cooker times vary for each slow cooker, so do check your muffins during the cooking time and look out for them being ready earlier or needing more time, and adjust the cooking time accordingly.
Check the muffins are cooked through by testing with a cake tester, metal skewer or toothpick in the centre of the muffin. Once cooked through, remove the silicone mould from the slow cooker and leave to cool on a cooling rack.Run a knife around each muffin then carefully push them out by pushing the bottom of the mould out. Leave the muffins to cool completely. If you have two moulds, now fill the second mould with the other half of the mixture as directed above. If you only have one, wash it up and reuse.Cook for 45 minutes on high (the second batch tends to take less time as the slow cooker is already hot) then check the muffins as above to see if they are done.