Pierce the skin of the sweet potatoes all over with a fork, spray with cooking spray and place in a 9-by-13-inch flameproof baking dish. Bake the sweet potatoes until tender, about 1 hour.
Cool the sweet potatoes enough to handle, and then cut each in half the long way and scoop out the flesh, leaving 1/8-inch of flesh around the skin. Reserve the skins for a later use.
In a medium bowl, add the sweet potato flesh, mascarpone, brown sugar, cinnamon, cardamom and vanilla seeds. Using an electric hand mixer or the bowl of a stand mixer fitted with a paddle attachment, smash until combined. Spoon the mixture into the sweet potato skins. Top each potato skin with three Modjeska halves, caramel-side down. Alternatively, place 4 caramel halves topped with 3 marshmallow halves on each potato skin.
Return the sweet potatoes to the baking dish and bake until the centers are warm and the Modjeskas start to melt, about 15 minutes. Turn on the broiler and cook until dark brown and bubbly, 1 to 2 minutes. Sprinkle the tops with sea salt to serve.