Heat the oven to 180C/fan 160C/gas 4. Oil a 20cm spring-form tin and line the sides and base with baking paper, making sure the paper comes a little higher than the sides of the tin.
Sieve the flour, cocoa, baking powder, and bicarb into a bowl. Add the sugar, a pinch of salt and the Horlicks.
Mix the biscuit spread, 1 tbsp vegetable oil, buttermilk, vanilla and egg in a jug, then pour into the bowl. Beat well until smooth, then pour into the tin (it will only come about 2cm up the tin).
Bake for 20-25 minutes until risen and a metal skewer comes out clean when poked into the centre of the cake. Leave to cool in the tin. Brush with the rum (if using) while the sponge is just warm, then leave to cool to room temperature.
For the mousse
Melt the chocolate in a bowl over (but not touching) simmering water (or in a microwave in short blasts), then leave to cool. Whip the cream in a large bowl until you get soft peaks, then fold in the cooled melted chocolate very gently trying not to knock any air out.
Spoon the mousse over the cooled cake in the tin, and smooth the top. Chill for at least 4 hours until the mousse has set.
For the ganache
Heat the cream in a pan until hand-hot. Remove from the heat and add the chocolate and butter.
Stir until the chocolate has melted, then tip into a bowl. Leave to cool to room temperature.
When the mousse has set, pour/spread the cooled ganache over the cake and cool for another 2 hours to set, or overnight if making it the day before.
Remove from the tin, transfer to a serving plate, and gently pull away the baking paper around the sides of the cake. Use a warm knife to cut slices of the cake, and serve scattered with the pretzel pieces over the top.