Place the custard into a microwave-proof bowl and heat up, stopping to thoroughly stir every 30 seconds, until it is very warm to the touch. Pour the chopped milk chocolate into the custard and keep stirring until the chocolate has completely melted and combined with the custard. Place this to the side to cool.
Slice the Swiss roll or chocolate cake into 1-centimeter pieces, ready to be layered through the trifle.
In a stand mixer, or using a handheld whisk, beat the whipping cream together with the powdered sugar and the Kirsch until it has thickened and holds its shape.
Start the trifle by placing a layer of the chocolate cake, using roughly half of the chopped pieces of cake, at the bottom of the trifle bowl.
Cover the cake with half of the chocolate custard, then top the chocolate custard with half of the whipped Kirsch cream.
Open the cherry pie filling and carefully spoon this into the center of the trifle, on top of the Kirsch cream.
Place the rest of the chocolate cake pieces on top of the cherry pie filling, then top the cake with the rest of the chocolate custard and then, finally, with the rest of the Kirsch cream.
Decorate the Black Forest trifle with fresh cherries and the chocolate shavings, and store in the fridge until ready for serving.