Preheat oven to 325°F (160°C). Grease a 10-cup (2.5 L) classic or fancy Bundt pan; dust with all-purpose flour and set aside.
In a medium bowl, whisk both flours, poppy seeds, baking powder, baking soda and salt. In a large bowl with electric beaters or in a stand mixer with a paddle attachment, beat butter with sugar until fluffy. Beat in eggs, 1 at a time; beat in lemon zest, 1 tbsp (15 mL) of the lemon juice and the vanilla. Stir flour mixture into butter mixture alternately with buttermilk, adding 2 additions of flour mixture and 1 of buttermilk.
Scrape into the prepared pan; smooth top. Bake in centre of preheated oven until cake tester inserted in centre comes out clean, 50 to 60 minutes. Let cool in pan on rack for 15 minutes. Remove from pan and let cool completely.
Place cake on serving platter. In a medium bowl, whisk icing sugar with remaining 2 tbsp (25 mL) lemon juice; drizzle evenly over cake. Let stand at least 10 minutes before cutting.
For Mini Lemon Poppy Seed Cakes, replace pan with 8 greased and floured 1-cup (250 mL) mini Bundt pans (or 3- x 5- inch/625 mL mini loaf pans); spoon batter into pans. Bake as directed for 20 to 30 minutes. Transfer pans to rack and let cool before removing cakes.
Per each of 16 servings: 259 calories, 4 g protein, 11 g fat (6 g saturated fat), 38 g carbohydrates, 2 g fibre, 50 mg cholesterol, 212 mg sodium