Preheat the oven to 375°F. Grease and line a tall, 9 × 5-inch Pullman-style loaf pan with parchment paper. A regular height loaf pan may overflow—if this is all you have, take care to fill the pan less than 1 inch from the rim, and save the excess for muffins or mini cakes. Note: I used a regular 9x5 loaf pan here and was able to still use all the batter)

If using untoasted black sesame seeds, toast the black sesame seeds over medium heat in a small dry skillet until a few seeds jump in the pan and they start to smell toasty (but not burnt!), 2 to 3 minutes. Place the toasted black sesame seeds in a food processor and blend until a finely ground powder forms, 1 to 2 minutes. Set aside.

In a medium bowl, whisk together, the sweet rice flour, oat flour, millet flour, salt, and baking soda. Set aside.

Melt the chocolate, either in the microwave in 15 to 30 second increments, stirring between each interval, until fully melted and smooth, or over a double broiler for about 5 minutes, stirring frequently. Once the chocolate is melted, set aside and let cool slightly.

In a large bowl, cream together the sugar, butter, and black sesame seed powder until fluffy, 2 to 3 minutes with an electric mixer on medium speed or 3 to 4 minutes by hand. Add the eggs and vanilla, beating well. Fold in the melted chocolate until just incorporated.

Alternate folding in the flour mixture and the hot water (or coffee), ¼ of the flour/water at a time, until the batter becomes smooth and liquid.

Pour the batter into the lined loaf pan, taking care not to fill the pan higher than 1 inch from the rim. 

Bake for 30 minutes, then reduce the heat to 325°F and continue to bake for about 30-40 minutes more minutes until it springs back when touched (Note: Cynthia's non gluten-free version bakes up in about 15-20 minutes less time on the second bake). The cake should still be relatively moist in the center, and a skewer or cake tester should not come out completely clean.

Remove from the oven and let cool completely before turning the cake out onto a plate. The cake may fall; this is normal, as it is so moist. Enjoy topped with matcha whipped mascarpone (below), whipped cream, or ice cream, if desired. The cake tastes even better on the second day.

Store any leftovers in an air-tight container in the fridge (though I left mine uncovered on the serving platter and it was still great!). Since this is a butter cake, leftover slices taste best when brought to room temperature for 20-30 minutes before serving.

In the bowl of a stand mixer fitted with the paddle attachment or a medium mixing bowl with an electric mixer, combine the cream, mascarpone, salt. Whip on medium-high until the mixture begins to thicken.

Add the sifted sugar and matcha powder and continue whipping until soft thick peaks form.