Preheat the oven to 165 degrees Celsius.
Add honey, Powidła - plum butter and apple sauce to a small pot and heat on low heat stirring continuously. The mixture should get slightly warm, not hot, and be well combined. Take off the heat and let it cool for a few minutes.
Meanwhile combine oat flour, rice flour, cacao powder, all spices, baking soda, baking powder and salt in a bowl.
Add eggs and sugar in a big bowl. Whisk until the eggs turn pale and the mixture becomes slightly fluffy. This takes a few minutes.
Add the honey mixture from the stove, stir gently until combined.
Next add milk and greek yogurt, stir until incorporated.
Add the dry ingredients to the wet, little by little, stirring just until combined in between. Do not over mix.
Pour the batter to the prepared loaf pan. Bake in the middle of the oven for 35-40 minutes, until an inserted skewer comes out clean.
Remove from the oven, let it cool for 10 minutes, then remove from the pan and place on a cooling rack.
When the cake is complete cooled slice it in half lengthwise, making sure it is evenly thick all over.
Spread Powidła - Polish plum butter over the bottom layer of the cake into an evenly thick layer. Cover with the top part of the cake.
Add cacao powder, cornstarch, sugar and milk to a small pot. Heat on low heat stirring regularly to avoid it from burning and lumps from creating.
Stir until the chocolate mixture thickens, then remove from the stove and let it cool for 1-2 minutes.
Pour over the cake, spreading it all over the top. If you want the chocolate ganache to cover the whole cake make a double batch of the chocolate ganache and spread on the sides as well.