Preheat the oven to 180C/350F and line a 12 hole cupcake tray with liners. Line a baking tray with grease proof paper/a silicone mat/Silpat.
Make the crumble topping: Place the butter, oats, flour and sugar into a medium sized bowl and rub between your fingers until you have a coarse breadcrumb texture with some larger lumps.
Place half the crumble mixture onto your lined baking tray and place in the oven for 10-15 minutes, whilst you make the cupcake batter. Stir the crumbles after around 5 minutes and return to oven. They are done when they look dry and are crisp to the touch. They won’t really colour much, this is fine. Just make sure you can’t see the “wet” butter, they should be dried out.
Make the cupcakes: place the butter and sugars into a large bowl or the bowl of your stand mixer and beat until light and fluffy, around 2 minutes on medium-high speed.
Add in the eggs and vanilla extract, scraping down the sides as necessary, and mix until well incorporated, around 1 minute.
Place the apple chunks, flour, baking powder, bicarb, salt (if using) and spices into a medium sized bowl and stir until all the apple chunks are coated in flour.
Tip into the egg mixture and add in the milk, mix on low-medium speed until well combined and there are no lumps of flour.
Divide the cupcake batter between the liners, I used a 2tbsp cookie scoop and filled it level. Fill each liner 2/3 of the way.
Sprinkle around 1tsp of the crumble topping onto the top of each cupcake and press very lightly so they stick in the top of the batter.
Place cupcakes in the oven for 18-22 minutes, until risen, golden and an inserted skewer into the centre comes out clean.
Transfer to a wire rack as soon as possible (I find this stops the liners peeling away from the sides) and leave to cool completely.
Once cool, place the butter into a large bowl, or the bowl of your stand mixer and beat until fluffy. Gradually add the icing sugar, beating in between each addition until incorporated, then add in the custard powder and vanilla extract and beat until well combined.
Add the milk 1tbsp at a time, if required, until you have a thick and creamy frosting which holds its shape, but isn’t too stiff to pipe.
Half fill a piping bag fitted with an open star nozzle with the custard frosting and pipe a swirl onto each cooled cupcake.
Sprinkle some of the baked crumbles onto each cupcake and then garnish with half a slice of apple and a sprinkling of cinnamon, if desired.
Apple Crumble Cupcakes will keep in an airtight container, in the fridge for 3 days. Bring them up to room temperature before serving.