Prep: Preheat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper.
Melt butter and chocolate: In a saucepan, melt butter and chocolate over low heat, stirring until smooth. Remove from heat and let cool slightly.
Mix wet ingredients: Whisk in both sugars, then the eggs and vanilla.
Add dry ingredients: Gently stir in flour, cocoa powder, and salt until just combined.
Layer & swirl: Pour half the batter into the pan. Drizzle half the caramel over it. Add remaining batter, then swirl remaining caramel on top with a knife.
Bake: Bake 30–35 minutes, until the center is just set. Sprinkle with flaky sea salt.
Cool: Let cool completely in the pan before slicing (trust me).
Notes
Want a deeper caramel flavor? Use dark caramel or add a pinch of espresso powder.
Double the batch in a 9x13-inch pan — just add 5–10 minutes to bake time.