To make the chocolate topping on your cake softer, you can try a few techniques. One option is to use a different type of chocolate that has a lower cocoa content. Dark chocolate with higher cocoa percentages tends to be harder when set. Opting for milk chocolate or a blend of milk and dark chocolate can result in a softer texture. Another method is to add a small amount of fat, such as butter or heavy cream, to the melted chocolate before spreading it on the cake. This can help create a smoother and softer consistency. Additionally, you can try gently heating the chocolate topping after it has set to soften it slightly. Be cautious not to overheat it, as chocolate can seize or become grainy. Experimenting with these techniques should help you achieve the desired softness for the chocolate topping on your cake.