In a low (300°F) oven or toaster, toast the bread for 3-5 minutes, or until it just turns the slightest blush of brown, not more. We just want to dry out the bread a little here.
In a wide bowl, whisk together all the ingredients for the batter. Place a slice of brioche into the batter, and flip it around so that both sides are wetted through. Let soak in the batter for 2-3 minutes.
As the bread soaks, heat a non-stick pan or skillet over medium heat, and plop in a tablespoon of butter. When the butter melts and sizzles, lift the soaked brioche out of the batter and gently place it into the pan. Cook it in the pan until just lightly browned on the bottom, careful not to caramelize it too much, as we’ll be browning it a little more after. This should take about 2 minutes each side. Repeat dunking and pan-frying with the remaining slices of brioche.
Sprinkle the extra caster sugar onto the bread (roughly 1/2 tablespoon per side of toast). Torch the sugar until it bubbles and turns a glassy brown, like the top of a crème brulee. Repeat on all 4 slices, before flipping them over and doing the same on all the flipside. If you don’t have a blowtorch, you can do this in a toaster or a broiler oven too, though it might not be as even.
Using a small sieve, dust a liberal amount of kinako (estimating 2 teaspoons per toast) onto each slice, and serve on a plate alongside the matcha ice cream.