Preheat your oven to 350°F (175°C). Line the bottom and sides of loaf pans with parchment paper, leaving a slight overhang for easy removal.
In a medium bowl, sift together the cake flour, matcha powder, and baking powder twice. This removes lumps from the matcha and ensures an even, vibrant distribution throughout the batter.
In a small bowl, whisk together the room-temperature simple syrup and dark rum. Set aside.
In a large mixing bowl (or the bowl of a stand mixer fitted with the paddle attachment), beat the unsalted butter until smooth and creamy. Add the granulated sugar and inverted sugar (or honey). Beat on medium-high speed until light, fluffy, and pale.
In a separate bowl, lightly whisk the room-temperature eggs. Slowly add the eggs to the creamed butter mixture in 4 separate additions, beating thoroughly after each addition. Tip: Ensure the eggs are truly room temperature to prevent the batter from curdling.
Add the sifted dry ingredients to the butter mixture. Switch to a flexible spatula and gently fold the ingredients together just until combined and no flour pockets remain. Do not overmix. Gently fold in the candied black beans and chopped chestnuts, if using.
Gently fold in the candied black beans and chopped chestnuts, if using.
Pour the smooth batter into your prepared loaf pan and level the top with your spatula. Place the pan in the oven and immediately drop the oven temperature to 325°F (160°C). Bake for 40 to 45 minutes.
The cake is done when a toothpick inserted into the exact center comes out completely clean.
While the cakes are still hot from the oven, generously brush the tops and sides with the rum syrup. However you do not need to use every last drop for this single loaf cake.
Allow the cakes to cool in their pans on a wire rack for 30 minutes. Once warm but manageable, lift them out, wrap them tightly in plastic wrap, and refrigerate overnight. Resting overnight distributes the moisture and matures the matcha flavor beautifully.
Before serving, bring the cake back to room temperature. Drizzle with melted white chocolate and garnish with extra candied black beans and chestnuts. Slice and enjoy with a fresh cup of tea!