Preheat oven to 350 F and grease one 8” round cake pan. (If you prefer not to split the cake later on, you can use two shallow, 8” round pans.)
Cream the butter and sugar in a medium-sized bowl.
Add the egg and beat until thoroughly combined.
Mix in vanilla extract (or your flavoring of choice).
In another bowl, sift together the flour and the cream of tartar.
Gradually add the flour mixture to the batter, beating until incorporated.
Dissolve baking soda in warm milk.
Stir the milk mixture into the batter to form a thick, spreadable paste.
Pour the batter into your prepared cake tin(s), spreading and smoothing it with a spatula until its surface is roughly level.
Bake for 25 to 30 minutes (15 minutes if using two cake pans), or until a toothpick inserted into the center of the cake comes out clean.
Remove cake from oven and let stand until the bottom of the pan has cooled to room temperature. Turn cake out onto a wire rack.
If you baked a single cake, split the cake using a cake leveler (or a kitchen knife).
Spread bottom half of cake with a generous amount of prepared custard (see recipe below).
Place second layer of cake atop bottom layer. Spread the top of the cake with custard. (I spread custard along the sides for a more modern look, but the recipe only calls for icing between layers and atop the cake.)
Decorate cake with dried or candied fruits as desired.