1. Set a rack in the middle of the oven and preheat to 350 degrees.

2. Combine the flour, baking powder, and baking soda in a bowl and stir to mix.

3. In the bowl of a heavy-duty mixer, beat the butter and sugar with the paddle on medium speed until soft and light, about 5 minutes. Beat in the eggs one at a time, beating until smooth after each addition.

4. Decrease the mixer speed to low and add half the flour mixture. Stop the mixer and scrape down the bowl and beater with a rubber spatula. Beat in the sour cream, then the remaining flour mixture.

5. Use a large rubber spatula to give the batter a final mix. Fold in the poppy seeds. Scrape the batter into the prepared pan.

6. Bake for about 1 hour, or until the cake is well risen and deep gold and a toothpick inserted in the center emerges clean.

7. While the cake is baking, prepare the syrup: Combine all the ingredients in a nonreactive saucepan and bring to a simmer, stirring occasionally to dissolve the sugar. Strain through a fine-mesh strainer into a measuring cup.

8. When the cake is baked, place it on a jelly-roll pan. Use a skewer to poke about 20 holes into the cake. Gradually pour the syrup all over the top of the cake until it is all absorbed. If any of the syrup leaks out of the springform pan, spoon it back over the cake. The cake absorbs the syrup rather slowly.

9. Transfer the cake to a rack and cool to room temperature.

10. Remove the pan sides and slide the cake, on the paper, onto a platter. Run a sharp knife between the cake and the paper and pull out the paper.