Preheat oven to 350 degrees F (175 degrees C). Line a muffin tin with paper liners.
Combine boiling water and oats in a small bowl; let stand for 20 minutes.
Combine 1/2 cup butter, 1 cup brown sugar, and white sugar in a large bowl; beat with an electric mixer until creamy, about 3 minutes. Add bananas, eggs, and vanilla extract; stir well to combine. Mix in oatmeal.
Sift flour, baking soda, cinnamon, and salt together in a bowl. Fold into the oatmeal mixture. Divide batter evenly among the muffin cups.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 20 minutes. Let cool in the tin, about 5 minutes.
Poke 3 deep holes into each muffin using the round handle of a small wooden spoon. Drizzle 1 tablespoon caramel-flavored condensed milk over each muffin. Let cool completely, about 15 minutes.
Mix 1 cup brown sugar, walnuts, coconut, 1/2 cup melted butter, and heavy cream together in a bowl to make topping. Sprinkle topping over muffins.