Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone baking mats and set aside.

Melt the butter (i1) and chopped semisweet chocolate (i2) together over a double boiler or in the microwave until smooth and glossy; if using microwave, heat in 20–30 second bursts and stir between intervals. If using, stir in the vegetable oil (i13) while the mixture is warm.

Whisk the granulated sugar (i3) and light brown sugar (i4) into the warm chocolate mixture until mostly dissolved and allow to cool for 5 minutes.

Add the eggs (i5), one at a time, stirring briskly after each addition so the eggs emulsify into the chocolate. Stir in the vanilla extract (i6) and optional espresso powder (i12).

Sift the cocoa powder (i7) and add it along with the all-purpose flour (i8), baking powder (i10) and salt (i9) to the chocolate-egg mixture. Fold gently with a spatula until there are no visible streaks of flour; do not overmix.

Fold in the semisweet chocolate chips or chunks (i11), reserving a small handful if you want to press pieces on top before baking.

Scoop the dough into 3-tablespoon mounds (or 1/4-cup for larger cookies) onto the prepared sheets about 2 inches apart. Use a wet fingertip (dipped into cold water (i19)) to gently smooth the tops for a glossy crust.

Press reserved chocolate chunks (i11) into the tops if desired and sprinkle a pinch of coarse sea salt (i16) on each cookie.

Bake one sheet at a time for 10–12 minutes for 3-tablespoon cookies or 12–14 minutes for larger mounds, until the tops are shiny and cracked but centers remain slightly soft.

Allow the cookies to cool on the baking sheet for 8–10 minutes so the centers set slightly, then transfer to a wire rack.

Dust lightly with powdered sugar (i14) if desired and serve warm with vanilla ice cream or whipped cream (i15).

Store cooled cookies in an airtight container at room temperature for up to 3 days or freeze. Reheat in a 300°F (150°C) oven for 5–8 minutes or microwave briefly to restore warmth.