In the bowl of an electric mixer, mix butter and sugar. Blend until light and creamy.
Add eggs one at a time, mixing well after each addition. Add vanilla and mix to combine.
Add flour, baking powder and salt. Mix well until thoroughly combined.
Add milk and mix well. Add cacao nibs and mix until thoroughly combined.
In a cast iron skillet on medium heat, melt butter for top layer. Remove from heat and layer plantain slices. Sprinkle nutmeg and brown sugar on top. Return to heat and cook until brown sugar melts, 3-5 minutes.
Pour and spread cake batter on top in an even layer. Place in oven and bake for 40-45 minutes, until a knife inserted comes out clean.
Leave to cool for 15 minutes. Flip over onto a plate. Brush the jelly on top. Leave to cool completely. Slice and serve.
Although I used pepper jelly, you can use a more neutral jelly like apricot if you prefer.