Preheat oven to 325°F. Grease a 9x13-inch baking pan.
In a large mixing bowl, add the butter, oil, sugar, vanilla, baking soda, baking powder, and salt and beat until smooth.
Add the eggs and beat until completely combined.
Stir in ½ the flour, then add ½ the sour cream and stir to combine. Stir in the rest of the flour, then add the remaining sour cream and stir to combine.
Add the cocoa powder and stir until smooth.
Fold in the zucchini and ½ cup chocolate chips. Transfer the batter to the prepared pan and spread evenly.
Bake for 30 minutes, or until the cake seems set.
Remove the cake from the oven, sprinkle the remaining chocolate chips over the top, and return to the oven.
Continue baking another 5 minutes, or until a cake tester inserted in the center of the cake comes out clean.
Remove the cake from the oven and use a rubber spatula to spread the chocolate chips into a smooth icing.
Cool the cake completely, on a wire rack, before slicing and serving.
Notes
Check the ingredients of your gluten free flour blend for xanthan gum. If your flour mix doesn’t contain xanthan gum, try adding ½ teaspoon to the recipe.
Use room temperature eggs and sour cream. Ingredients at room temperature will mix more easily into the batter.
The zucchini should be grated but not drained. Many baking recipes call for squeezing the excess water out of the zucchini, but in this case the water adds moisture to the cake.