Preheat oven to 350 degrees F (175 degrees C). Grease a donut pan.
Whisk cassava flour, ground ginger, baking powder, and salt together in a bowl. Beat butter and 2 tablespoons coconut sugar together in a separate bowl using an electric mixer until fluffy; add coconut milk, maple syrup, 3 tablespoons molasses, and egg and beat until smooth.
Mix cassava mixture into butter mixture until batter is thick and just combined. Transfer batter to a pastry bag or plastic bag with a corner snipped. Pipe batter into the prepared donut pan.
Bake in the preheated oven until a toothpick inserted in the center comes out clean, 10 to 12 minutes. Transfer donuts to a cooling rack and cool completely, at least 15 minutes.
Place 2 tablespoons coconut sugar and crystallized ginger in a spice grinder; process until ginger is powdery. Pour mixture into a small bowl.
Brush tops of donuts with the remaining 2 tablespoons molasses. Press donuts, molasses-side down, into coconut sugar-ginger mixture until coated.