Preheat the oven to 350°F (175°C). Generously grease a 12-inch cast iron skillet with butter, making sure to coat all curves and the surface.
In a large bowl, whisk together the gluten-free flour mix, dark brown sugar, cocoa powder, baking soda, baking powder, and kosher salt until evenly combined.
Add the lightly beaten eggs and olive oil to the dry ingredients and mix just until combined; do not overmix.
Fold in the grated zucchini and date crumbles until evenly distributed through the batter.
Pour the batter into the prepared cast iron skillet and smooth the top with a spatula.
Bake for 50–60 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs.
Allow the cake to cool in the pan. Once cooled, dust the top with powdered sugar if desired and serve.