Preheat the oven to 180°C/350°F. Lightly grease 2 x 8 in/20cm round cake tins and line the bases with parchment paper.
Add the cocoa into a large bowl (or the bowl of a stand mixer), then pour in the boiling water and stir until smooth.
Add the remaining cake ingredients and mix until combined, (by hand or with an electric whisk or stand mixer). The batter will be quite runny.
Divide the batter between the prepared tins and bake for 30 minutes or until an inserted skewer comes out clean. Allow to cool in the tins for 10 minutes, then transfer to a wire rack to cool completely.
For the black pepper buttercream:
Use a pestle and mortar to grind the Schwartz black pepper grounds until they are very fine.
In a large bowl, cream the butter until fluffy. Add in the icing sugar, Schwartz pepper and vanilla and beat well. If necessary, add more milk, a tsp at a time, to make it a thick frosting consistency and beat again.
Once the cakes have cooled, spread one with the strawberry jam then place the other cake layer on top. Spread the top and sides with the buttercream and decorate with the strawberries and shaved dark chocolate.