Preheat a fan oven to 165C and line a cupcake pan with paper liners. Place 1 regular oreo in the base of each cupcake case.

Mix 1 tsp instant coffee with 90ml freshly boiled water and leave aside.

In a large mixing bowl, sieve the flour, cocoa, baking powder, and baking soda together.

Add in the brown and white sugars and salt, and mix well.

In a measuring jug, whisk together the oil, buttermilk, egg, and vanilla. Pour into the dry ingredients and mix into a thick batter.

Pour in the slightly cooled coffee into the batter and mix again until thin and runny.

Pour the mixture into the cupcake liners, filling them 3/4 full.

Bake for 18-20 minutes until a skewer comes out clean. Allow to cool completely before icing.

In a large bowl, whip the room temperature butter until pale and fluffy.

Gradually add the icing sugar on low speed until fully incorporated, then mix on high speed for 1 minute before adding vanilla.

Gradually add in the milk until the buttercream is thick but spreadable.

Add yellow gel food colouring gradually, mixing until desired colour is achieved. Let sit in the fridge for 1-2 days for deeper colour.

Remove buttercream from the fridge, bring to room temperature, and mix again to remove air bubbles.

Smooth melted chocolate over the top of each cupcake and refrigerate until set.

Fill a piping bag with buttercream and practice piping on greaseproof paper.

Pipe the buttercream onto the cupcakes, pressing a mini oreo into the center.

Store cupcakes in the fridge for up to 3 days, allowing to come to room temperature before serving.