In a medium bowl, mix together oat flour, almond flour, and cocoa powder. Add maple syrup, stir to mix. Add coconut oil and salt, stir to mix.
Transfer the mixture to a 9-inch (23 cm) tart pan (see note 3). Use your fingertips to press the mixture firmly into the bottom and sides of pan, distributing the mixture evenly to fill in any thin spots.(If dough sticks to your fingers, wet your hands with cold water). Bake until crust no longer looks wet, about 12 minutes.
Meanwhile, warm coconut milk in a small saucepan over low heat. Once simmering, remove from heat. Whisk in chocolate until melted. Add mint extract and salt. Whisk to combine. Set aside to cool slightly.
Let the crust cool for 5 to 10 minutes, then re-whisk chocolate filling for 30 seconds (or until smooth). Pour filling into cooled crust. Refrigerate until set, about 2 hours.
If garnishing, spoon coconut whipped cream over tart and top with candy canes and/or mint (optional). Slice and serve. Cover and refrigerate extras.