YIELD: one 8-inch cake
Whisk everything but the butter together. Add the butter and work together with your hands until it forms crumbles. Set aside in the freezer.
Preheat oven to 350 F. Grease an 8-inch square cake pan and line with parchment so there is overhang on two sides.
In a medium bowl, whisk dry ingredients.
In a small bowl, whisk together warm milk and kopiko powder. Set aside.
In a large bowl, beat the butter and sugar until fluffy with a hand mixer. Beat in the egg and vanilla. Beat in the coconut milk mixture on low speed. Add the flour and mix on low speed until just combined.
Add half of the batter (318g) to the prepared pan. Sprinkle with half (167g) kopiko crumble. Dollop remaining batter on top (this makes it easier to spread but it will still be a little fiddly) and spread the best you can. Top with remaining kopiko crumbles.
Bake until a toothpick comes out clean, about 45 minutes.
Let cool in pan for 15 minutes. Lift out of the pan and slice into squares. Serve warm or at room temperature.