Make cake: Preheat oven to 350ºF. Grease a 9"-x-13" pan with cooking spray and dust with cocoa powder.
cooking spray, cocoa powder
Prepare chocolate cake mix according to package directions and bake until a toothpick inserted in the center comes out clean, 25 to 30 minutes. Let cool completely.
1 box chocolate cake mix
Poke cake all over with the handle of a wooden spoon. In a small bowl, mix together sweetened condensed milk and melted chocolate. Pour mixture into the holes.
1 14-oz can sweetened condensed milk, 1 cup semisweet chocolate chips
Make frosting: In a large bowl using a hand mixer, beat butter, powdered sugar, cocoa powder, vanilla, and salt. Beat in heavy cream, adding more by the tablespoon until consistency is creamy but can hold peaks.
1 cup butter, 2 1/2 cup powdered sugar, 3/4 cup cocoa powder, 2 tsp pure vanilla extract, pinch of kosher salt, 1/4 cup heavy cream
Spread frosting all over cake with an offset spatula and sprinkle with chocolate chips.
1/2 cup mini chocolate chips