Prepare the Cinnamon Sponge:

1. Preheat your oven to 350°F (175°C).
2. Grease and line an 8-inch square baking dish with parchment paper.
3. In a large bowl, whisk together the flour, granulated sugar, baking powder, baking soda, ground cinnamon, and salt.
4. In another bowl, mix the dairy-free milk, vegetable oil, vanilla extract, and apple cider vinegar until well combined.
5. Add the wet ingredients to the dry ingredients, mixing until just combined.
6. Pour half of the batter into the prepared baking dish and spread it evenly.

Add the Cinnamon Sugar Layer:

1. In a small bowl, mix the granulated sugar and ground cinnamon.
2. Sprinkle half of the cinnamon sugar mixture evenly over the batter in the baking dish.

Add the Remaining Batter:

1. Pour the remaining batter over the cinnamon sugar layer and spread it evenly.
2. Sprinkle the remaining cinnamon sugar mixture on top of the batter.

Bake the Cake:

1. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
2. Allow the cake to cool in the pan for 10 minutes.

Prepare the Spiced Chocolate Sauce:

1. In a medium saucepan, heat the dairy-free milk over medium heat until warm.
2. Add the dark chocolate chips, cocoa powder, granulated sugar, ground cinnamon, ground nutmeg, and vanilla extract, stirring continuously until the chocolate is melted and the sauce is smooth.
3. Remove from heat and let it cool slightly.

Serve:

1. Pour the spiced chocolate sauce over the warm cake.
2. Serve the cake warm and enjoy the gooey, cinnamon-spiced goodness.