PAN: Mix butter and flour in a small bowl, then brush inside a 10-cup Bundt pan.
BATTER: Rub sugar and orange zest in a large bowl with hands to release the natural oil. Whisk in flour, baking powder and salt.
WHISK eggs, oil, orange juice, buttermilk and vanilla in a bowl. Stir into flour mixture until just combined. Pour into prepared pan.
BAKE until a tester inserted into centre of cake comes out clean, 35 to 40 min. Cool for 20 min in pan. Remove cake to a rack and cool completely, about 40 min.
GLAZE: Place chocolate in a small bowl. Heat cream in a small saucepan over medium-high, until bubbles start to form. Pour over chocolate and let stand 1 min. Stir until smooth. Stir in honey, then drizzle over cake.