Make the cupcakes: Preheat the oven to 350 degrees F. Line a 12-cup muffin pan with liners. Combine the butter and unsweetened chocolate in a large microwave-safe bowl; microwave in 30-second intervals, stirring, until melted and smooth. Let cool slightly, then whisk in the granulated sugar and eggs until smooth. Add the flour, baking powder, baking soda and salt; whisk until just combined. Add the milk and whisk until smooth, then stir in the banana and peanuts.
Divide the batter among the prepared muffin cups, filling each about three-quarters of the way. Bake until the tops of the cupcakes spring back when gently pressed, about 20 minutes. Let cool 5 minutes in the pan, then transfer to a rack to cool completely.
Meanwhile, make the frosting: Beat the butter, confectioners¿ sugar and salt in a large bowl with a mixer on medium speed until combined. Add the vanilla and milk; increase the speed to medium high and beat until light and fluffy, about 3 minutes. Add the crushed sugar cones and beat 1 more minute. Transfer to a piping bag fitted with a large star tip. Arrange the cupcakes on a baking sheet; pipe tall swirls of frosting onto the cupcakes. Refrigerate until the frosting is firm, about 20 minutes.
Meanwhile, make the chocolate shell: Combine the semisweet chocolate and coconut oil in a microwave-safe bowl; microwave in 30-second intervals, stirring, until melted and smooth. Let cool slightly, then pour into a short wide drinking glass for dunking the cupcakes. One at a time, turn each cupcake upside down and dunk the frosting in the melted chocolate; rotate to coat and let the excess drip off. Return to the baking sheet and decorate with sprinkles while the chocolate is still wet. Refrigerate until set, at least 30 minutes or overnight.