Blend the oatmeal into a powder.
Mix oat flour, cocoa powder, sweetener, and salt in a bowl.
Add coconut oil and plant-based milk, then knead into a dough.
Press the dough firmly into a tart mold.
Bake in a preheated oven at 180°C for 15 minutes.
Melt soy milk and dark chocolate together using a double boiler or microwave.
Pour the chocolate filling over the baked tart.
Chill in the refrigerator for at least 1 hour until set.