Preheat oven to 350°F. Grease a 9x13 inch baking pan. Prepare cake batter according to package directions and pour into prepared pan.

Bake for 25-30 minutes until toothpick inserted comes out with few moist crumbs. Remove from oven.

Immediately poke holes all over cake surface using wooden spoon handle, spacing about 1 inch apart. Don't pierce through bottom.

In microwave-safe bowl, combine sweetened condensed milk and chocolate bars. Microwave 30 seconds, stir, then 15 seconds more until smooth.

Pour chocolate mixture over warm cake. Use spatula to guide filling into holes. Cool on counter 1 hour.

Refrigerate cake 2 hours. Meanwhile, chill mixing bowl in freezer.

Whip cold heavy cream until thickening. Add powdered sugar, cocoa powder, and vanilla. Beat until stiff peaks form.

Spread chocolate whipped cream over chilled cake. Drizzle with chocolate syrup and sprinkle mini chocolate chips on top.

Refrigerate until ready to serve. Slice with sharp knife dipped in hot water for clean cuts.