Preheat oven to 350°F.

In a large bowl, mix butter and sugar until light and fluffy. Add vanilla extract, eggs and milk and stir together just until well combined.

In a separate large bowl, add flour, 1 1/3 cups of the chocolate chips, baking powder, baking soda and salt. Stir to combine and then pour the wet ingredients into the dry ingredients bowl. Mix just until combined.

Add the butter, brown sugar and maple syrup to a medium saucepan. Bring to a boil and cook over medium-high heat for 3-5 minutes until thickened, stirring constantly. Remove from the heat and stir in vanilla and salt.

Spray a 9x13 pan with non-stick cooking spray or grease with butter.

Pour half of the cake batter in the 9x13 pan. Drizzle half of the caramel over the batter. Top the caramel with the remaining half of the cake batter and finish by pouring the remaining caramel sauce on top.

Use a butter knife and drag through the cake and caramel mixture a few times in each direction to swirl the caramel into the cake batter.

Top with the remaining chocolate chips and bake uncovered at 350°F for 45-50 minutes, until the center of the cake is set and a toothpick comes out clean with no batter.