Preheat the oven to 180°C. Line a muffin tin with 12 paper cups and set aside. Place the butter and chopped chocolate in a heatproof bowl and either melt in the microwave or over a saucepan of simmering water, stirring until smooth. Set aside and let cool down a bit.
In a large bowl, beat the eggs with the brown sugar and vanilla extract until light and fluffy. Add the cooled butter and chocolate mixture and mix in. In a second bowl, mix together the flour, cocoa, baking powder, baking soda, and salt, then add to the large bowl alternating with the buttermilk and amarula and mix until just combined. Divide the batter between the 12 paper cases and then bake for 16-18 minutes. Test with a toothpick to see if the dough is still sticking and only remove the cupcakes when the toothpick comes out clean. Remove from the oven and let cool completely on a wire rack.
For the chocolate sauce, roughly chop the chocolate, then place in a microwave-safe bowl with the butter, sugar, cream and Amarula and microwave in several short intervals until the chocolate is melted and mixes together to form a smooth sauce. If you don’t have a microwave, you can do this over a pot of simmering water. Set aside and let cool.
For the buttercream, put the room-warm (!) butter in a large bowl and beat on the highest setting for about 5 minutes until the butter is very light and fluffy. Sift the cocoa into the bowl and mix well. Add the vanilla extract and salt and stir. Gradually add the sweetened condensed milk to the bowl while stirring – beating on the highest setting until you get a fluffy cream. Fill into a piping bag with a star tip.
Using a cupcake corer or sharp knife, poke a hole in the center of the cupcakes, fill in some Amarula Chocolate Sauce, then pipe a ring of buttercream onto each cupcake. A little more chocolate sauce in the resulting cavity fillings (may also overflow over the edge) and then decorate with chocolate balls.