For the cupcakes: Firstly, pre-heat the oven to Gas Mark 4/180C and line muffin tins with 12 cases.
Whisk the cocoa powder, flour, bicarb, baking powder and salt together in a large bowl until combined, then set aside. In a medium bowl; whisk the eggs, caster sugar, brown sugar, oil and vanilla together until completely smooth. This will take a couple of minutes, and the mixture will turn slightly paler in colour.
Pour half of the wet ingredients into the dry, and then half of the buttermilk. Whisk gently, then add the remaining wet ingredients and the buttermilk. Whisk until just combined - the batter will be a little thin and lumpy!
Spoon the mixture into the muffin cases about two thirds full each and bake for 18-21 minutes. Once baked, put the muffins onto a wire rack to completely cool.
For the salted caramel drizzle: Place the sugar, golden syrup and water in a heavy-bottomed pan and stir to combine.
Heat over a high heat, occasionally swirling the pan, until it turns a medium gold amber colour - this will take about 10 minutes. Once the caramel starts to deepen in colour, remove the pan from the heat. Slowly and carefully whisk in the cream, the mixture will bubble up so stand clear and keep stirring.
Add the butter, whilst continuing to stir until melted. Then add the salt and vanilla to combine, I added around 2 large pinches. Pour into a heat-safe container (a pyrex bowl, jug or a jam jar will do) and set aside to cool and thicken.
For the frosting: Beat the butter on a high speed until creamy. Then slowly add the icing sugar, mixing until its thoroughly combined and smooth. Do this in stages, so your kitchen doesn't get entirely covered in sugar!
Then add the Biscoff spread to the buttercream and beat until smooth. Finally, add the milk bit by bit, continuing to beat until you achieve the desired consistency. Set aside until ready to use.
To assemble: Prepare a piping bag with a tip of your choice. I used a 2D/large star tip, and pipe swirls on top of each cupcake. Sprinkle over the crushed Lotus Biscoff biscuits. Pour the salted caramel into a piping bag, and snip off the end. Pipe the caramel over the cupcakes following the lines of the frosting or in any pattern you like.