Preheat oven to 350°F (180°C). Butter a 9-inch round cake pan. Line bottom of pan with parchment paper.

In the bowl of a stand mixer fitted with the whisk attachment, beat egg yolks and ½ cup (100 grams) sugar at medium-high speed until fluffy and pale yellow in color, 1 to 2 minutes, stopping to scrape sides of bowl. With mixer on medium-low speed, beat in ⅓ cup (80 grams) coconut milk, 1 tablespoon (15 grams) room temperature water, and 1 teaspoon (7 grams) cream of coconut, beating just until combined and stopping to scrape sides of bowl.

In a medium bowl, whisk together flours, baking powder, and ¾ teaspoon (2.25 grams) salt. Fold flour mixture into egg yolk mixture until combined. (Mixture will be thick.) Transfer to a large bowl.

Clean bowl of stand mixer and whisk attachment. Using the whisk attachment, beat egg whites and cream of tartar at medium-high speed until foamy. Gradually add ½ cup (100 grams) sugar in a slow, steady stream, beating until stiff peaks form. Stir one-fourth of egg white mixture into batter until lightened and well combined; add remaining egg white mixture in three additions, folding until combined and no streaks or lumps remain. (Batter will start out thick but become lighter, smoother, and fluffier with each addition.) Pour batter into prepared pan. Gently tap sides of pan to help settle batter.

Bake until a wooden pick inserted in center comes out clean, 20 to 26 minutes. Let cool in pan for 10 minutes. Invert cake onto a wire rack; discard parchment. Place a 12-inch rimmed serving plate on bottom of cake layer; turn cake, right side up, onto plate. Using a wooden pick, poke holes all over top of cake.

In a medium bowl, whisk together condensed milk, ⅓ cup (80 grams) cold cream, rum (if using), remaining ⅔ cup (203 grams) coconut milk, remaining ⅓ cup (98 grams) cream of coconut, and remaining ¼ teaspoon salt. Gradually pour and spread milk mixture all over top and sides of warm cake, allowing some to soak in before adding more. Lightly cover and refrigerate until chilled, at least 3 hours or up to overnight.

When ready to serve, use a slotted spoon to transfer and spread crushed pineapple in an even layer on top of cooled cake, leaving a ½-inch border.

Clean bowl of stand mixer and whisk attachment. Using the whisk attachment, beat remaining 2 cups (480 grams) cold cream and remaining 1 tablespoon (12 grams) sugar at medium-high speed until medium-stiff peaks form. Dollop whipped cream on top of pineapple; using the back of a spoon, spread and swirl as desired. Swirl any pineapple juice that runs into milk mixture until well combined. Garnish with toasted coconut, if desired.