Lemon Curd (if making from scratch):

1. Combine Ingredients:
   1. Beat the butter and sugar in the bowl of an electric mixer until creamy.
   2. Add the eggs one at a time, beating until each is combined.
   3. Beat in the lemon zest.
   4. Stream in the lemon juice with the mixer on medium-low speed. The mixture will look curdled.
2. Cook Lemon Curd:
   1. Transfer the mixture to a microwave-safe bowl.
   2. Microwave for 5 minutes, stirring every 1 minute.
   3. Then microwave for 1 to 2 more minutes, stirring after each 30-second increment, until it coats the back of a spoon.
3. Chill:
   1. Place a piece of plastic wrap over the bowl, pressing it directly onto the curd.
   2. Refrigerate for 2 to 4 hours, or until chilled and thick.

Lemon Crumb Cake:

1. Preheat Oven and Prepare Pan:
   1. Preheat the oven to 350°F (175°C).
   2. Spray a 9×9 inch baking dish with nonstick baking spray.
2. Make Crumb Topping:
   1. Whisk together the flour, sugar, lemon zest, and salt in a bowl.
   2. Add the cold butter and rub it into the flour with your fingers until the mixture resembles sand.
3. Combine Dry Cake Ingredients:
   1. In a separate bowl, stir together the flour, baking soda, and salt.
4. Cream Butter and Sugar:
   1. In the bowl of an electric mixer, beat the softened butter and sugar together on high speed until creamy, about 3 to 4 minutes.
   2. Beat in the egg yolks one at a time.
   3. Beat in the vanilla and lemon extracts and lemon zest until combined.
5. Add Wet and Dry Ingredients:
   1. Beat in half of the flour mixture until just combined.
   2. Beat in the Greek yogurt.
   3. Beat in the remaining dry ingredients until just combined.
6. Beat Egg Whites:
   1. In a separate bowl, beat the egg whites with an electric hand mixer until foamy.
   2. Gradually add the 2 tablespoons of sugar, beating until stiff peaks form.
7. Fold in Egg Whites:
   1. Fold the egg white mixture into the cake batter.
8. Assemble Cake:
   1. Spread the batter into the greased baking dish.
   2. Dollop the lemon curd on top and gently swirl it in with a knife.
   3. Sprinkle the crumb mixture all over the batter.
9. Bake:
   1. Bake for 40 to 45 minutes, or until a tester inserted in the center comes out clean.
10. Cool and Serve:
    1. Remove from the oven and sprinkle the top with powdered sugar.
    2. Let cool slightly before serving.