How to Make It
Preheat oven to 350°F. Beat butter on medium speed with a heavy-duty electric stand mixer until creamy. Gradually add sugar, and beat until light and fluffy, 3 to 5 minutes. Add eggs, 1 at a time, beating just until blended after each addition. Stir in liqueur and zest.
Add flour and tonic water alternately to butter mixture, beginning and ending with flour, beating on low speed just until blended after each addition. Pour batter into a greased (with shortening) and floured 10-inch Bundt pan.
Bake in preheated oven until a long wooden pick inserted in center comes out clean, 1 hour to 1 hour and 10 minutes. (Tent with aluminum foil after 45 to 50 minutes to prevent excessive browning, if necessary.)
Cool in pan on a wire rack 15 minutes; invert cake onto rack, and cool completely, about 30 minutes. Spoon Lemon-Elderflower Glaze over cake.