Heat oven to 325 degrees F. Grease and flour 9-inch bundt pan. In medium-size bowl, combine flour, cocoa powder, baking powder and salt.
In 2-1/2- to 3-quart saucepan, combine water, bourbon, butter and coffee powder. Over medium heat, stir bourbon mixture just until butter melts. Remove saucepan from heat. Whisk in granulated sugar. Let cool for 10 minutes.
Whisk in eggs and vanilla. Gradually whisk in flour mixture until combined. Pour into prepared bundt pan.
Bake in 325 degree F oven for 60 minutes or until wooden pick inserted in center comes out clean. Let cake cool in pan on wire rack 15 minutes. Invert cake onto wire rack; remove pan. Let cool completely.
In medium-size bowl, stir together the confectioners sugar, corn syrup and enough milk until glaze is smooth and pourable. Place baking sheet under rack with cake. Pour glaze over cake, using spatula to help cover the cake completely. Drizzle melted semisweet chocolate over top of cake, if desired. Refrigerate cake until glaze is set, about 15 minutes. With 2 spatulas, transfer cake to cake plate. Refrigerate, covered, for up to 3 days.