• Preheat the oven to 350°F and line an 8x8 baking pan with parchment paper and non-stick cooking spray.

Make the crumb topping:

• In a small bowl, use a fork to mix together the melted butter, brown sugar, granulated sugar, salt, vanilla and lemon zest. Once combined, stir in the flour until a thick crumb mixture forms. Set aside.

Make the cake:

• 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.