Preheat oven to 350°F (175°C) and lightly grease a 9-inch cast-iron skillet.
In a large bowl, whisk the melted butter, dark brown sugar, and granulated sugar until smooth.
Add the room temperature egg and vanilla extract, whisking vigorously for 1 minute until emulsified.
Sprinkle the almond flour, tapioca starch, baking soda, and kosher salt over the wet ingredients. Fold gently with a spatula until just combined.
Fold in 1 cup of chocolate chunks. Press the dough evenly into the prepared skillet. Scatter the remaining 1/4 cup of chunks on top.
Bake for 20-22 minutes until edges are golden and the center is slightly soft. Cool in the pan for 15 minutes before serving.