Bloom the cocoa powder. In a large bowl, add the coffee. Sift the cocoa powder on top. Whisk until well combined and no clumps remain. Set aside for the flavors to bloom.
Prep the pan and pre-heat the oven. Meanwhile, preheat the oven to 350F/176C. Also, grease the baking pan by adding a little butter (or oil, to keep it dairy-free) and spreading around with a pastry brush. Make sure to grease the sides of the pan and the corners.
Add wet ingredients. Add the eggs, coconut milk, melted butter and vanilla extract. Whisk until well combined.
Add dry ingredients. Add the sugar, glutinous rice flour, salt and baking powder. Whisk until well combined and there are no lumps, a good 2-3 minutes. Add to the greased baking pan, making sure to scrape the bowl clean with a spatula. The batter will be runny and loose.
Add the chocolate bar. On a cutting board, roughly chop the chocolate bar. It's ok if it's not uniform in size. You want some chocolate shards and some chunky pieces. Carefully sprinkle the chopped chocolate evenly over the top of the batter, making sure to include all the little bits and shards.
Bake. Carefully transfer the baking pan to the preheated oven. Bake until the edges are puffed and pulling away from the pan and the middle is set, about 70-75 minutes. There should be no jiggle in the middle when you shake the edge of the pan.
Cool. Remove from the oven and cool, at least 1-2 hours. Sprinkle with flaky sea salt. Cut into squares and enjoy.