Heat oven to 350°F. Line a 9×13-inch pan with parchment, leaving overhang.
Cream softened butter and sugar for the shortbread until light and smooth.
Blend in vanilla and salt, then mix in flour just until a soft, crumbly dough forms.
Press dough evenly into the pan. Dock lightly with a fork.
Bake 18–22 minutes, until edges are lightly golden. Cool in pan 10 minutes.
Cook caramel: melt butter, brown sugar, condensed milk, corn syrup, and salt in a saucepan over medium heat, stirring constantly.
Simmer 4–6 minutes, stirring, until thick and 238–240°F or deeply golden. Remove from heat and stir in vanilla.
Pour caramel over the warm crust. Spread evenly and chill 15–20 minutes to set.
Melt chocolate with oil in a microwave or double boiler, stirring until silky.
Spread chocolate over caramel. Sprinkle flaky salt if using. Chill until firm, about 20 minutes.
Slice into bars with a warm knife. Wipe the blade between cuts for clean edges.