Get Ready: Preheat your oven to 350°F (175°C). Lightly grease or spray a 9x13 inch baking pan with non-stick spray.
Mix Wet Ingredients (Cake): In a large mixing bowl, beat the softened butter and granulated sugar together until they're light and fluffy. Stir in the vanilla extract, eggs, and milk just until everything is combined. Don't overmix!
Combine Dry Ingredients (Cake): In a separate large bowl, whisk together the flour, 1 ⅓ cups of the dark chocolate chips (save the rest for topping!), baking powder, baking soda, and a pinch of salt.
Bring it Together: Gently pour the wet ingredients from step 2 into the dry ingredients bowl. Mix just until the flour streaks disappear – again, avoid overmixing for a tender cake!
Make the Caramel: In a medium saucepan, combine the butter, brown sugar, and corn syrup. Bring this mixture to a boil over medium-high heat, stirring constantly. Let it boil for 3-5 minutes until it thickens nicely. Remove from heat and stir in the vanilla extract and salt.
Layer it Up: Pour half of your prepared cake batter into the greased 9x13 inch pan. Drizzle half of your warm caramel sauce evenly over this batter layer.
More Layers: Carefully dollop the remaining cake batter over the caramel. Then, pour the rest of the caramel sauce on top.
Create Swirls: Take a butter knife and gently drag it through the cake and caramel layers a few times in different directions to create beautiful swirls.
Final Touch & Bake: Sprinkle the remaining dark chocolate chips over the top of the swirled cake. Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean.
Enjoy! Let your delicious coffee cake cool slightly before slicing and serving.