Make the cake: Center a rack in the oven and heat oven to 350 degrees. Butter and flour an 8 1/2-inch loaf pan. Place the pan on 2 stacked baking sheets or an insulated baking sheet.

Zest and juice the lemon(s). Whisk together the flour, baking powder and salt in a small bowl. Put the sugar in a large bowl, add the lemon zest, and rub together with your fingers until the sugar is moist and aromatic. Add the eggs, one by one, whisking each vigorously before adding the next. Whisk in the vanilla and half the lemon juice, and then whisk in the heavy cream until smooth.

Add the flour mixture in 3 additions, using the whisk to gently stir the dry ingredients into the batter. When the flour is incorporated, add the butter in 2 additions, again stirring gently with the whisk. You should have a thick, smooth, shiny batter.
Switch to a flexible spatula and stir in the poppy seeds. Scrape the batter into the pan.

Bake until the cake has risen and cracked along the center and, most important, a tester inserted deep into the cake comes out clean, 60 to 70 minutes. Take a look at it after about 45 minutes and tent it loosely with foil if it’s getting too dark too fast.

Transfer the pan to a rack, cool for 5 minutes, and then run a table knife between the cake and the sides of the pan. Unmold the cake, then turn it right side up onto the rack. Cool to room temperature.

Make the glaze, if you like: Stir together the confectioners' sugar and half of the rest of the lemon juice until smooth. If needed, add more juice, a drop at a time, until you have an icing that falls slowly from the tip of a spoon. Spread it over the cooled cake to coat evenly, sprinkle with extra poppy seeds and let stand until set. Wrapped well, the cake will keep at room temperature for about 4 days; unglazed, it can be frozen for up to 1 month.

https://cooking.nytimes.com/recipes/1022856-poppy-seed-tea-cake