In a small bowl, whisk together the flour, baking powder, baking soda and salt. Set aside.
In a stand mixer fitted with the paddle attachment, cream the softened butter and sugar on medium-high speed until light and fluffy, about 2 minutes.
Add the eggs, one at a time, mixing for about 1 minute after each addition. Then mix in the vanilla, sour cream, lemon juice, lemon zest and poppy seeds.
Add the flour mixture and mix on low speed until just combined. Do not over-mix the batter.
Pour into the pan use a spatula to spread it evenly. Top with the crumb mixture, using your hands to press the mixture into chunks. Lightly press the topping on top of the batter.
Bake at 350 for 35-40 minutes, until a toothpick inserted into the center of the cake comes out clean or with just a few moist crumbs.