I realise that this is technically a four milk cake with the addition of buttermilk in the batter, but I didn't want to confuse the issue, split hairs, gild the lily, or discombobulate you in any way.
For the Cake:
150g (1 cup) plain flour
40g (2 heaping tablespoons) cocoa powder
2 teaspoons baking powder
1/4 teaspoon fine sea salt
1 teaspoon ground cinnamon
115g ( 1 stick) unsalted butter, at room temperature
160g (3/4 cup) brown sugar
1/2 teaspoon vanilla extract
125ml (1 2/ cup) buttermilk
For the Milk Soak:
125ml (1/2 cup) condensed milk
125ml (1/2 cup) evaporated milk
125ml (1/2 cup) full cream milk
25g ( 1 oz) dark chocolate, chopped
For the Topping:
200ml (1 cup) whipping cream
100ml (1/2 cup) sour cream
Make the Cake:
Preheat the oven to 180C. Butter a 8cm x 8cm cake pan and dust with a heaping tablespoon of cocoa powder. Shake the pan so that the cocoa powder sticks to the butter and coats the base and sides of the pan. Shake off any excess cocoa powder. This will form a non-stick surface and make the cake easier to remove from the pan. Set aside.
Sift the flour, cocoa, baking powder, salt and cinnamon into a mixing bowl. Set aside.
In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and sugar for 5 minutes on medium speed until fluffy. Add the eggs one at a time, mixing well after each addition. Add the vanilla and mix again to combine.
Add one third of the flour mixture and half of the buttermilk to the butter mixture and mix at low speed until just incorporated. Add another third of the flour and the remaining buttermilk and mix again until just incorporated. Add the remaining flour and mix briefly.
Pour the batter into the prepared pan and bake for 30 - 35 minutes, until a skewer inserted into the centre comes out clean.
Make the Milk Soak:
Put all the milk soak ingredients into a small saucepan and put over low heat until the chocolate has melted. Leave the milk soak to cool for 10 minutes. Once the cake comes out of the oven, poke hole all over it with a skewer, going right down to the pan. Pour the milk soak over the hot cake. Set aside to cool to room temperature. Once cooled, chill in the refrigerator overnight until cold and the milk soak has been absorbed by the cake.
Make the Topping:
In the bowl of an electric mixer fitted with the whisk attachment, whip the cream and sour cream until firm peaks. Dollop it onto the cold cake and spread evenly to the edges with a spatula. Chill in refrigerator until ready to serve.