In a medium heavy saucepan, combine everything but the vanilla. Melt together over low heat, stirring gently. Once everything is combined, increase the heat to medium and bring to a boil. Reduce the heat slightly and boil for 10 minutes without stirring. Watch it closely and reduce heat as needed to keep the bottom from burning. Remove from heat and gently stir in the vanilla. Set aside.

Preheat oven to 375. Line a 9x13 pan with tin foil (leave enough to hang over the sides so you can pull the bars out after baking) and set aside.

Whisk together dry ingredients and set aside.

With a mixer (or by hand, which is what I did this time since I have no mixer, stand or otherwise) beat the butter, both sugars, egg and vanilla, about 2-3 minutes.

Add the dry ingredients and chocolate chips and stir to combine. Press just over half of the dough into the prepared pan. Pour half of the caramel sauce over the dough, then sprinkle with the Twix pieces. Evenly drop the rest of the dough in bits over the top of the Twix.

Bake for 18-20 minutes until the top is golden brown (don't overbake it!). Let cool on a wire rack, then carefully remove from pan and slice into squares.

Serve with vanilla ice cream and top with the remaining caramel sauce. Sprinkle with coarse sea salt, if desired.