Preheat the oven to 160C/325F and spray a 2lb loaf tin with cooking spray. Take a thin strip of grease proof paper with enough length to have overhang either end of the pan and place along the middle. This will make it easier to life the cake out later.
Place the butter and sugar into a large bowl, or the bowl of your stand mixer and beat until light and fluffy, around 2 minutes on medium-high speed.
Add in the eggs, vanilla and kirsch, if using, scrape down the sides of the bowl and mix until well incorporated, around 1 minute on medium speed.
Pour in the cooled melted chocolate and mix briefly to combine.
Add in the flour, cocoa powder, baking powder and salt and mix until it begins to come together, then add the milk.
Mix on medium speed until everything is well combined and smooth. Tip in the tinned cherries and fold in by hand.
Tip the batter into your prepared loaf tin and smooth into an even layer. Place into the oven for 60-70 minutes, until risen and an inserted skewer into the centre comes out clean.
Leave to cool for 10 minutes before taking a skewer and poking holes all over the top of the cake. Take your reserved cherry juice and spoon it over the cake. You may not need it all, just slowly spoon it over and wait for it to absorb. I used about 7 spoonfuls of cherry juice - you want the cake to be moist, but not soggy with cherry juice.
Leave to cool completely before lifting the cake out onto a plate to decorate.
Place the double cream and sugar into a large bowl and whisk until firm peak. You want to be able to pull the whisk out and it holds the peak, but the tip bends over.
Either spread the cream over the top of the cake, or place in a piping bag and pipe swirls over the top. Place fresh cherries on the top and sprinkle shaved chocolate over the cream. If you have any cherry juice left, you can also spoon a little bit over the cream.
Slice and serve!
Black Forest Loaf Cake will keep covered, in the fridge, for 3 days. Without the cream topping, the cake will keep in an airtight container, at room temperature for 4 days, so you can make this ahead of time and just decorate it when required!