Bake the chocolate cake 

Soak the finely diced prunes in boiling water to cover. Set aside.

Meanwhile, preheat the oven to 350°F. Grease two 8-inch round pans and coat the insides lightly with cocoa powder, tapping out excess.

In the bowl of a stand mixer, combine the sugar, gluten free all purpose flour, cocoa, baking powder, baking soda, and salt. Mix until completely combined.

Drain the prunes and add them to the mixing bowl as well as the eggs, milk, oil, and vanilla. Mix for 2 minutes on medium speed. Scrape down the sides then stir in the boiling water with a spatula. The batter will be thin.

Divide the batter evenly between the two pans.

Bake for 35 - 40 minutes in the preheated oven, until a toothpick inserted in the center of the cake comes out clean. Cool in the pan on a wire rack for 10 minutes, then remove the cake from the pan and cool the rest of the way on the cooling rack.

Make the chocolate icing 

Beat the softened butter on medium speed for 2 minutes in a stand mixer fitted with the paddle attachment until nice and creamy.

Add the powdered sugar, cocoa powder, milk, vanilla, and pinch of salt. Beat on low speed for 30 seconds, then increase the speed to high and beat for 1 full minute.

If the chocolate icing is too thick add a bit more milk or if it is too thin add a bit more powdered sugar until you reach the desired consistency for icing this cake.