Line the insert of a 4- or 6-quart slow cooker with a large piece of foil, then generously butter the foil.
Whisk together the melted butter, sugar, cocoa powder, flour, eggs, vanilla and 1/2 teaspoon salt in a medium bowl. Fold in the chocolate chunks. Scrape the batter out into the prepared insert in an even layer. Cover and cook on low for 3 hours; the cake should be set around the edges and gooey in the center.
Serve the cake warm, scooped into a bowl and topped with ice cream.