Preheat oven to 160C. Grease and line a 20cm cake pan with baking paper or grease an 8 cup, cupcake pan.
Combine the cocoa, coconut flour, baking powder, cinnamon and sea salt. Add the eggs, honey, vanilla and oil. Mix well until combined.
Spoon mixture into your prepared pan. Bake cake for 40 minutes or cupcakes for 20-25 minutes. Remove from the oven and cool.
FROSTING: Melt chocolate in a medium heatproof bowl over a saucepan of simmering water until smooth. Cool slightly. Cream butter, icing sugar and vanilla essence together. Add egg yolk and melted chocolate and beat until fluffy. Decorate cake.