Step 1

Heat oven to 350°F. Line an 8x8-inch metal baking pan with parchment or grease with oil.

Step 2

In a microwave-safe bowl, melt the unsweetened chocolate in 30-second intervals in the microwave, stirring between each until just melted and smooth.

Step 3

Whisk in the olive oil and sugar.

Step 4

Add cocoa powder and espresso powder. Whisk until combined.

Step 5

Add eggs, egg yolk and vanilla. Whisk well - at least 100 strokes to help form a crackly crust.

Step 6

Add flour and salt. Stir until combined.

Step 7

Stir in chocolate chips.

Step 8

Spread batter into prepared pan. Bake 20-25 minutes, or until just barely set. They will look underdone when you pull them out but will set as they cool.

Step 9

Sprinkle with flaky salt and cool before cutting.