Preheat oven to 325°F (170°C).

In the bowl of a stand mixer fitted with the paddle attachment, beat butter and sugars at medium speed until fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Add eggs, one at a time, beating well after each addition. Beat in vanilla.

In a medium bowl, whisk together flour, salt, and baking soda. With mixer on low speed, gradually add flour mixture to butter mixture alternately with buttermilk, beginning and ending with flour mixture, beating just until combined after each addition.

Spray a 5-cup Bundtlette pan with baking spray with flour. Divide batter among wells (about 107 grams each). Tap pan on a kitchen towel-lined counter several times to settle batter and release any air bubbles.

Bake until a wooden pick inserted near center comes out clean, 15 to 20 minutes. Let cool in pan for 15 minutes. Invert Bundtlettes onto a wire rack, and let cool completely.

In the top of a double boiler, combine dulce de leche, milk, and coconut extract. Cook over simmering water, stirring occasionally, until fluid. Pour warm dulce de leche mixture over cooled cakes to fully coat. Press toasted coconut into tops and sides of cakes. Drizzle with melted chocolate. Refrigerate in an airtight container for up to 3 days.