1. MAKE THE CRUMB TOPPING: In a bowl, combine the flour, granulated sugar, brown sugar, baking powder and salt. Add the oil and stir until combined. Using your fingers, pinch the mixture together to form crumbs.
2. MAKE THE CAKE: Preheat the oven to 350°. Grease a 9-inch springform pan with the butter and dust with flour. In a small pitcher, combine the buttermilk and vanilla. In a medium bowl, whisk together the flour, baking powder, baking soda, salt and mace. In a large bowl, beat the sugar and oil for 1 minute. Add the eggs, 1 at a time, beating well after each addition. Beat in the flour mixture at low speed in 3 batches, alternating with the buttermilk, just until the batter is smooth.
3. Scrape the batter into the prepared pan and scatter the crumbs on top. Bake the cake in the center of the oven for about 1 hour, or until a toothpick inserted in the center comes out clean. Transfer the cake to a rack to cool slightly.
4. MAKE THE GLAZE: In a small bowl, combine the confectioners' sugar with the bourbon; add up to 1/2 teaspoon of water if the glaze is too stiff. Using a small spoon, drizzle the glaze over the warm cake. Let cool before unmolding.
ONE SERVING Calories 399 kcal, Total Fat 16.7 gm, Saturated Fat 1.7 gm
Make Ahead: The cake can be wrapped in plastic and kept at room temperature for up to 5 days or frozen for up to 1 month.