If you want to use dulce de leche to fill cakes or cookies, it has to be thick. Here's a trick for achieving the right consistency in just a few minutes!
Pour the dulce de leche sauce into a medium saucepan and place over medium-high heat.
Stirring constantly, bring the mixture to a boil for 5 to 8 minutes or until the desired consistency is reached (if you get to the point where the mixture is bubbling like thick, hot lava, your caramel will be very thick).
Transfer the mixture to a deep bowl and then blitz it with an immersion blender to get rid of any lumps (if you have any), while the mixture is still warm.