Preheat oven to 350F. Melt chocolate in a double boiler over hot water; stir frequently. Take off heat. Lightly mix eggs, 1/4 cup milk, and vanilla in a bowl. In a large mixing bowl mix dry ingredients on low speed for 30 seconds to combine. Add butter and remaining milk. Mix on low speed until dry ingredients are moistened. Now beat on medium speed for 1 1/2 minutes; scrape down sides. Add egg mixture in 3 parts, beat for 20 seconds after each part; scrape down sides. Add melted chocolate and beat to incorporate.
Grease, line and flour three 9” round cake pans or bake in one and level later. Fill batter into pans. Bake for 25-35 minutes (more for 1 pan). Cakes are ready when an inserted cake tester (tooth pick) comes out clean. Let cakes cool in pan for 10 minutes, then loosen the sides with a small metal spatula and invert onto greased wire racks.
Combine cream, sugar, and butter in a saucepan and bring to a boil on medium-high heat. Stir occasionally until sugar is dissolved.
In a small bowl, mix together cornstarch, vanilla, and 1 tablespoon cold water until cornstarch is fully dissolved. Add to cream mix, bring to a boil, and simmer until thickened, about 1 minute.
Place coconut in a food processor. Pulse until coconut is finely chopped. Remove cream mixture from heat and stir in coconut until well combined. Transfer to a large baking dish; let cool.
Cover filling with plastic wrap and chill in fridge overnight. Just before using, place mixture in the bowl of an electric mixer with paddle attachment. Beat until smooth and creamy, 4 to 5 minutes.
Level cake into three even layers. Place first layer in a spring form pan. Spread about 1/4 of coconut filling. Place second cake layer on top. Spread another 1/4 of the filling and finish with the last cake layer. Spread remaining filling on top and let it also flow down the sides.
Chill for a couple hours. Before removing from spring form, run a knife along the edge to loosen from pan.