Preheat oven to 180°C (350°F). Lightly grease a 9-inch (23 cm) ovenproof skillet with 1 tablespoon vegetable oil, then place skillet in oven for 5 minutes to warm while you prepare the dough.

Cream butter and sugars

In a medium mixing bowl, beat the softened unsalted butter (113 g) with the granulated sugar (65 g) and light brown sugar (75 g) until mixture is smooth and slightly fluffy, about 1–2 minutes by hand or 1 minute with an electric mixer on medium speed.

Add egg and vanilla

Beat in the large egg until fully incorporated, then stir in 1 teaspoon vanilla extract. Scrape down the bowl to ensure an even mixture.

Combine dry ingredients

In a separate bowl, whisk together the all-purpose flour (150 g), baking soda (1 tsp) and salt (1/2 tsp) until evenly distributed.

Form cookie dough

Gradually add the dry ingredients to the butter-sugar mixture and stir until just combined; do not overmix. Fold in the semisweet chocolate chips (135 g) until evenly dispersed.

Bake in skillet

Carefully remove the hot skillet from the oven. Press the cookie dough into the warm skillet in an even layer, smoothing the top with a spatula. Bake immediately in the preheated 180°C (350°F) oven for 18–22 minutes, until edges are golden brown and center is set but still soft (a toothpick will come out with a few moist crumbs).

Rest briefly

Remove skillet from oven and place on a heatproof surface. Let the cookie rest 5 minutes to firm slightly; it will remain warm and gooey in the center.

Chocolate Ganache

While the cookie bakes (or up to 15 minutes before serving), heat the heavy cream (120 ml) in a small saucepan over medium heat until it just begins to simmer and small bubbles form at the edge—do not boil.

Place the chopped semisweet chocolate (120 g) in a heatproof bowl. Pour the hot cream over the chocolate and let sit 1 minute to soften, then gently whisk from the center outward until smooth and glossy. Stir in the optional butter (15 g) and a pinch of salt until incorporated. Keep warm; if it thickens, rewarm gently over a double boiler or briefly in the microwave (5–10 seconds) and stir.

To Serve

Spoon or drizzle warm chocolate ganache over the center of the skillet cookie while it is still hot.

Top with 2 scoops of vanilla ice cream placed directly on the warm cookie so it begins to melt. Sprinkle a pinch of sea salt flakes over the finished dessert if desired.

Serve the skillet on a trivet or heatproof surface with spoons and a knife for slicing. Encourage sharing immediately while warm and gooey.