Cake:

Preheat the oven to 350 degrees. Butter and flour two 9-inch cake pans. In the bowl of an electric mixer, beat the butter and sugar together until it is light and fluffy. Add the eggs one at a time, beating well in between each addition, and continue to beat until very light and fluffy. Add buttermilk and mashed banana and mix well. In a separate bowl, sift and dry ingredients together (flour, baking soda, salt, baking powder), then add to batter and mix at low speed on the electric mixer until moistened. Beat one minute at medium speed, then fold in pecans and pour batter evenly into the two prepared cake pans. Bake for 25 to 35 minutes, until a toothpick inserted in the center comes out clean. After removing the cakes from the oven, allow them to cool for five minutes, then invert onto a cooling rack. Assemble when thoroughly cooled.

Coconut Custard:

Line a baking sheet with parchment paper and cut another piece of parchment to the same size. Set both aside. Combine the coconut milk and milk and stir together. In a bowl, mix 2/3 cup of sugar with 1/3 cup cornstarch and stir with a fork until free from lumps. Mix the sugar and cornstarch together with the egg yolks and 1 cup of coconut milk mixture. Whisk until smooth and set aside. In a heavy saucepan, bring the remaining coconut milk mixture to a boil. Pour it into the bowl with the egg yolk-sugar mixture, whisking constantly. Return the mixture to the saucepan over medium flame and continue stirring. Cook until small bubbles start to form on the surface of the custard. Remove from the heat and spread onto the parchment-lined baking sheet. Cover the top of the custard with the other sheet of parchment to prevent a skin from forming. Cool in the refrigerator.

To assemble Cake:

In a large bowl, gently toss banana slices with the lemon juice, and sugar. Trim the top of the first layer of cake so that the surface is level, then spread with a thin layer of custard. Place bananas completely over bottom layer, so that no cake is showing. Spread with another layer of coconut custard. Place next cake layer on top of this and spread the top generously with coconut custard. Garnish liberally with toasted coconut flakes.