Preheat oven to 350°F. Lightly grease a 9-inch cast iron pan with coconut oil. Mix the ground flaxseed with water with a fork or small whisk. Let rest for a few minutes.
In a medium bowl, combine the cassava flour, cacao powder, baking soda and sea salt.
In a large bowl, whisk together the coconut oil, applesauce, peanut butter, maple syrup, coconut sugar and vanilla extract until smooth. Add the flaxseed and water mixture until combined.
Add the flour mixture to the wet ingredients and stir until combined. Fold in the half of the chocolate chips, reserving some to sprinkle on top.
Transfer the mixture to the prepared skillet, lightly smoothing the top, and sprinkle the remaining chocolate chips on top. Bake in the oven for 20-25 minutes or until just set in the center. Remove and allow to cool for abut 10 minutes, then serve warm with vegan vanilla ice cream, if desired.
Notes
Let the skillet cookie cool before cutting it into slices.
Store leftovers covered at room temperature for the first day. Store covered in the refrigerator for up to 3 days.