Preheat the oven to 160ºC, fan 140ºC, gas mark 3 then grease and line 2 x 20cm round cake tins with parchment. Melt the chocolate in a microwave or in a bowl set over a pan of barely simmering water, stirring occasionally. Set aside to cool slightly.
Beat the butter, Light Muscovado Sugar and vanilla paste, using electric beaters or a freestanding mixer, until pale and ﬂuffy. Add the egg a little at a time, slowly beating until each bit is incorporated fully. Slowly pour in the chocolate, beating all the time. Next slowly beat in half the ﬂour until just combined, then the other half until just combined (don’t overbeat). Divide between the tins and bake for 35-45 minutes; they should be well risen but with a slight wobble. Cool completely in the tins.
For the ganache, melt the chocolate and cream together in a bowl in a microwave, or in a bowl set over a pan of barely simmering water, stirring occasionally. Gently mix in the Orange Extract and the liqueur if using, then set aside to cool slightly.
Beat the butter until smooth and creamy using electric beaters or a freestanding mixer. Add half the Icing Sugar, beat on slow until combined then turn up the speed to fast and beat for a minute until smooth and creamy. Repeat with the remaining Icing Sugar. On slow speed add the cooled orange ganache, beating gently until combined.
To assemble the cake, turn out the sponges and trim off the crusty top to level. Place one cake onto a plate or stand and spread a 0.2cm layer of buttercream on top, levelling it off with a palette knife. Top with the second sponge, cut-side down.
Using a palette knife, generously spread the frosting all around the sides of the cake ﬁrst (this allows you to hold the top of the cake still while you work around the cake). Spread a nice even layer over the top of the cake, moving the palette knife in a back and forth circular motion to create rough style swishy patterns. Serve as it is or decorate simply with chocolate curls or slices of candied orange.