Melt in the microwave: chocolate, oil, and maple; mix well until the mixture is smooth and homogeneous; add vanilla and rum, mix, and set aside.
In a small bowl dilute potato starch with two tablespoons of the milk, and mix well. Pour the rest of the milk in the cup to a small saucepan, add the diluted potato starch, cocoa powder, xanthan gum, and salt. Cook on medium heat, while stirring, until it boils; lower the heat and cook until thickened. Remove from heat.
Add the chocolate mixture we melted earlier, and mix well until fully combined; This step may take a few minutes. Let cool for a few minutes.
Pour the chocolate cream on the baked crust, level with a spatula, and refrigerate for at least two hours until the cream sets; During the summer, it is better to chill it in the fridge overnight.