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Preheat the oven to 350°F. Grease an 8" square pan with nonstick spray or softened butter. Line with a long piece of parchment paper extending up and over two opposite sides of the pan to create a sling. (For details on how to make an 8" round cake, see "tips," below.)
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To make the batter: In a large bowl, beat together the butter, brown sugar, and vanilla for 30 seconds.
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Beat in the eggs, followed by the buttermilk, then the mashed banana.
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Beat in the baking soda, then the salt.
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Use a flexible spatula to gently fold in the flour, stirring just until the last streak disappears; be careful not to overmix. Scrape the batter into the prepared pan.
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To bake the cake: Bake the cake for 25 to 30 minutes, rotating the pan at the halfway point. Bake until a toothpick or thin paring knife inserted into the center comes out with only a few moist crumbs clinging to it.
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Remove the cake from the oven and let it cool in the pan for about 20 minutes, or until you can safely lift it out by the parchment sling without burning yourself. Run a table knife around the edges if the cake resists. Let cool to room temperature before frosting.
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To make the frosting: In the bowl of a stand mixer fitted with the flat beater, beat the butter and cream cheese on medium-low speed until well combined.
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Beat in the vanilla and salt, then gradually stir in the confectioners' sugar.
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Stir in the cocoa powder followed by the malt powder. Beat on medium speed for 30 seconds.
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Generously frost the top of the cake (we like leaving the sides bare), and garnish with the crushed candy. Note: If you’re making the cake ahead and refrigerating it, don’t garnish until just before serving.
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Storage information: Store the cake, covered, in the refrigerator for up to three days.
