Preheat the oven to 400℉ (200℃) conventional (see note 2 if using a convection oven). Lightly grease a 10-inch cast iron skillet and set aside.
In a small bowl, sift together the flour, baking soda, and salt. Using a whisk, mix until well combined. Set aside.
In a medium bowl, combine the melted butter, white granulated sugar, and soft light brown sugar. Using a whisk, mix until smooth and glossy.
Add the egg and vanilla and whisk until well combined and the mixture looks slightly thickened and cohesive.
Add the pre-sifted dry ingredients into the wet mixture and using a spatula, gently fold until a soft cookie dough forms. Be careful not to overmix and only mix until the flour is just incorporated.
Add the roughly chopped semi-sweet chocolate, reserving about 2 tablespoons of the larger chocolate chunks for the top. Gently fold the chocolate into the dough until just combined.
Transfer the cookie dough into the prepared skillet and press it into an even layer, smoothing out the top. Scatter the reserved chocolate chunks evenly over the surface.
Bake for 12–13 minutes, or until the edges are lightly golden and set, while the top is slightly puffed and soft to the touch but set enough that no raw batter sticks to your finger. Be careful not to overbake. The centre should still look soft and gooey, with a slight shine on the surface, but it should not look wet or raw.
Allow the pizookie to rest in the skillet for about 10 minutes. This allows it to finish setting while keeping the centre soft and gooey. Top with a scoop of vanilla ice cream and serve warm. Enjoy!