1. Combine the starter with 100 grams of the whole wheat flour and 175 grams of the coconut milk in a bowl to make the jumpstarter. Stir with a rubber spatula or big spoon until the jumpstarter comes together in a loose dough or a thick batter; no traces of dry flour should be seen. Cover the bowl with plastic or a damp kitchen towel and let it sit at room temperature overnight, anywhere from 8 to 12 hours. (You can refrigerate the jumpstarter after fermenting to hibernate for up to 24 hours; let it come to room temperature before proceeding.) Meanwhile, feed the remaining starter: Add 50 grams each whole wheat flour and water and stir or shake. Cover and return it to the refrigerator.
2. An hour before you’re ready to bake, take the butter out of the fridge to soften.. Grease a 9-inch deep-dish pie plate, square baking pan or baking dish, or 9-inch springform pan with extra butter. Combine the remaining 150 grams flour with the baking powder and salt in a small bowl.
3. Heat the oven to 350°F. Put the stick of butter in a large bowl with the sugar and beat with an electric mixer or whisk vigorously until fluffy, 2 to 3 minutes (the sugar won’t dissolve). Beat or whisk in the eggs one at a time. Beat or whisk until creamy, 2 to 3 minutes. (It’s okay if the sugar isn’t completely dissolved.)
4. Fold in the jumpstarter and the flour mixture with a rubber spatula or big spoon until just combined. Stir in 75 grams of the coconut. The batter will be a little stiff but not quite as firm as cookie dough; add a little more coconut milk if necessary.
5. Spoon the batter into the prepared pan and bake until a toothpick inserted in the center comes out clean, 50 to 60 minutes. While the cake bakes, put the reserved coconut milk in a small saucepan over medium-low heat; you should have about 200 grams. Add 20 to 30 grams turbinado sugar (depending on your taste) and cook without boiling, stirring occasionally, until the sugar dissolves and the milk thickens a little, about 10 minutes. Spread the remaining 50 grams of coconut into a small rimmed baking sheet and put in the oven with the cake; toast until lightly browned, about 5 minutes. Remove the toasted coconut and save for decorating.
6. When the cake comes out of the oven, poke it all over with a fork and pour the coconut milk on top while it’s still hot. Scatter the toasted coconut on top. Serve warm or at room temperature, with ice cream or whipped cream. (Cover any leftover cake and refrigerate for up to 2 days. Or store individual slices tightly wrapped in the freezer for up to a couple months, and warm them in the microwave before serving.)

— Recipe adapted from Bittman Bread