Make the pudding with the 2 3/4 cups milk (slightly less than what is called for on the package) according to the manufacturer's instructions. I cooked mine in the microwave for about 8 minutes, stirring after every minute, until it was thick.
Cover the pudding with plastic wrap pressed directly on to the surface so it doesn't form a skin. Allow pudding to cool to room temperature. Set your bowl inside a larger bowl of ice or cold water if you want to speed it up. Stir periodically.
Beat the butter and icing sugar until blended and add the 1/4 cup of rum or liqueur. Add the cooled pudding 1 spoonful at a time, beating all the while.
Cut the cake into 3 layers if using a tube pan. Spread the cherry preserves on the bottom layer of the cake. Top with the next layer. Transfer about 1/2 a cup of butter cream to a piping bag with a large tip and reserve. Spread 1/2 the remaining butter cream over the second layer. Place the top layer of the cake. Spread the remaining butter cream evenly over the top and sides as well as the inside hole if you used a tube pan.
Sprinkle the top and sides of the cake with the Krokant. Pipe 12 rosettes of butter cream evenly around the top of the cake. Place 1 cherry in each rosette.
Keep cake chilled until ready to cut and serve.