Preheat oven to 375°F (190°C). Grease and flour a 10-in. (25 cm) Bundt cake pan. In a small bowl, mix together banana and lemon juice. Set aside. In a medium bowl, mix together flour, cocoa, baking soda and salt. Set aside. In a large bowl, beat oil and brown sugar until light and fluffy. Beat in eggs, one at a time and add vanilla. Blend in half the flour mixture and half the buttermilk, then mix in remaining flour and buttermilk. Fold in bananas and 1/2 cup (125 mL) of the chocolate chips. Spread into prepared pan.
Bake until a skewer inserted into the thickest part of the cake comes out with only a few moist crumbs, about 55 min. Cool in pan, set on cooling rack, for 30 min. before turning out onto a cake plate.
To make glaze, melt remaining 1/4 cup (60 mL) chocolate chips with milk in a small microwaveable bowl on HIGH for 1 min., then whisk together until smooth. Drizzle over cake. Serve warm or at room temperature.
Naturally low in fat, buttermilk gives this cake a moist and tender crumb, while mashed bananas make a moist replacement for butter. *Add 30 min. cooling time to total cooking time to get total time of 105 min.