In a medium saucepan, combine sweetened condensed milk, unsalted butter, light brown sugar, and light corn syrup over medium heat. Stir continuously for about 5-7 minutes until thickened and caramel colored. Remove from heat and cool.

Preheat oven to 350°F (175°C). In a mixing bowl, blend room temperature unsalted butter, all-purpose flour, powdered sugar, and fine salt until crumbly. Press mixture into bottom of an 8x8 inch baking pan. Bake for 22-25 minutes until lightly golden. Cool completely.

In a large bowl, whisk granulated sugar, cocoa powder, flour, and pinch of fine salt. In another bowl, beat eggs, vegetable oil, and vanilla until smooth. Fold dry ingredients into wet mixture until combined. Pour brownie batter over cooled shortbread crust.

Bake at 325°F (163°C) for 38-43 minutes until set with slight jiggle in the center. Cool in pan for at least 30 minutes before adding ganache.

Heat heavy whipping cream over medium-low heat until bubbling, pour over dark chocolate chips. Let sit for a minute, stir until smooth. Mix in unsalted butter and optional corn syrup.

Once brownies are cooled, pour caramel layer on top, spread evenly. Pour chocolate ganache over caramel, smooth with spatula. Sprinkle flaky sea salt if desired, refrigerate for at least 1 hour to set.

Lift brownies from pan using parchment edges, cut into squares with a warm knife. Serve at room temperature and enjoy.