For the frosting: Combine the granulated sugar and 1/4 cup water in a small saucepan and bring to a boil. Cook, swirling the skillet, until the syrup turns deep amber brown (don't stir!). Remove the pan from the heat and pour in the cream. Once the bubbling subsides, stir the cream until the caramel dissolves and you have a thick caramel sauce.
Refrigerate until chilled, at least 1 hour.
For the cake: Meanwhile, preheat the oven to 350 degrees F. Butter two 9-inch round cake pans then line the bottoms with parchment paper. Butter and flour the paper.
Whisk the flour, cinnamon, baking soda and salt in a bowl until evenly combined, then add the coconut and walnuts and toss again. In another bowl, whisk the bananas, oil, both sugars, vanilla, eggs and beer until smooth. Pour the wet ingredients over the dry ingredients and whisk until the batter just comes together. Divide the batter between the prepared pans, then bake until a cake tester inserted in the center of the cakes comes out clean, 30 to 35 minutes.
Cool the cakes in their pans for 30 minutes, then invert them onto a cooling rack and remove the parchment paper to let them cool completely.
Remove the caramel sauce from the refrigerator and let stand for 20 minutes to come to room temperature. Beat the butter for the frosting with an electric mixer on medium speed until lightened and creamy, about 3 minutes. Add the confectioners' sugar, poppy seeds, vanilla and salt and beat until evenly incorporated. Add the caramel sauce and beat until the frosting is fluffy and light, about 3 minutes.
Put a cake layer on a cake plate, then spread about 1 cup of the frosting over it with an offset spatula or butter knife. Top with the second cake layer, then scrape the remaining frosting on top. Spread it evenly over the top and sides. Refrigerate the cake until the frosting has firmed, at least 30 minutes or up to overnight, before serving.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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