Grease or insert parchment paper into a 9×13 pan.
Melt the butter and chocolate together. Mix in sugar and vanilla. In a different bowl, mix Mochiko flour, cacao powder, baking powder, cinnamon, and cayenne. Beat in 1/2 of the Mochiko flour mixture, eggs, and coconut milk. Add in another 1/2 of the Mochiko flour mixture, 11/2 cup milk and beat until all flour and milk is added and mixed in until smooth.
Pour into pan and cook at 350 deg F for 45-55 minutes.
Let cool completely. Cut with a plastic knife or wet your knife between cuts.