In a large bowl, using an electric mixer on low speed, cream butter for about 15-30 seconds.
Add maple syrup and brown sugar, continuing to cream until well combined.
Add egg and egg yolk and continue to cream until combined.
To the large bowl add cornstarch, baking powder, baking soda, salt, plus 1/4 cup cassava flour. Mix until combined. Continue to add the remaining cassava flour in 1/4 cup increments, mixing in between additions until combined.
Last, mix in dark chocolate chips and dried cranberries.
Cover dough and place in the refrigerator for 20-30 minutes (up to overnight).
Preheat oven to 375°F. Scoop out about 1/4 cup of dough, roll into a ball, then slightly press down with the palm of your hand and bake for 10 minutes.
Let cookies cool completely (or at least for 20 minutes) on the baking sheet before transferring to a plate.
If desired, drizzle with melted dark chocolate. To melt chocolate, microwave 2 tablespoons chocolate chips and 1/8 teaspoon oil in 30 second intervals, stirring in between, until completely melted.
Store cookies in an airtight container. You may also freeze for 6 months.