How wet is this cake after the malted milk mixture is absorbed?

When I made it, I did not think the cake was wet by any means. Moist, yes- but it's not soaking and dripping off, if that's what you fear. It's a sort of sponge cake, which means it will hold it's share of whatever liquid you care to pour on. Kind of struck me as a malted milk version of tres leches- with less milk variety. I would not put the dry milk powder in the cake itself because you are already adding in buttermilk. It would be an extra dry ingredient without any extra liquid to offset it, and the texture would suffer as a result. Although you can add a tablespoon or so of milk powder to a cake mix to add richness, in this case I don't think that is needed. Adding the powder to the cake mix could also dilute the delicate flavor of the malted milk so it would be lost in the final cake. Adding it as a soaker liquid allows it to shine though. The milk is also being mixed with the white chocolate after it melts, and I'm not sure that would work without the milk to help carry it into the cake crumb. Just be sure you let the cake chill in the fridge several hours- and I find soaked cakes do best chilled overnight, honestly.