Celebrate Passover with a chiffon cake so tender and airy, you'll be tempted to make it year-round (even if you don't observe the Jewish holiday). To keep it kosher, whipped egg whites act as the only leavener, and coconut oil replaces butter.
Preheat oven to 350 degrees. In a small bowl, whisk together matzo meal, potato starch, and salt. In a large bowl, beat egg whites on medium-high to soft peaks. Gradually add 3/4 cup sugar and beat until stiff (but not dry) peaks form. In another large bowl, beat egg yolks with remaining 3/4 cup sugar and vanilla seeds until pale and doubled in volume, about 5 minutes. Gradually beat in 3 tablespoons oil, then coconut milk, just until combined. Add matzo mixture; beat just until combined. Stir in one-third of egg whites, then gently fold in remaining whites in two additions, just until combined and no streaks remain (do not overmix). Transfer to an ungreased 10-inch tube pan (preferably footed) with a removable bottom; smooth top with an offset spatula.
Bake cake until golden on top and a tester inserted in center comes out clean, 40 to 45 minutes. Invert pan and let cool completely. If pan is not footed, invert center of tube onto the neck of a bottle to allow air circulation.
Turn cake right-side up. Run a thin, sharp knife around sides of pan and center tube. Lift cake by center tube; remove ring. Run knife around bottom of pan to loosen; invert cake to remove. Place on a plate or stand. Combine remaining oil and chocolate in a bowl. Place over (but not in) a pot of simmering water; stir until melted. Remove from heat and place over an ice-water bath, stirring constantly, until thickened slightly but still pourable, 1 to 2 minutes. Pour frosting evenly over top and sprinkle with coconut. Cut into wedges; serve.
The frosting should be pourable, like brownie batter. If it gets too thick, microwave itfor 10 seconds at a time to thin it.