Combine the flour, sugar, lemon zest, and salt. Add the butter and combine using a pastry blender or a fork. Mix until crumbly.
Cover and chill.
To make the cake:
Preheat oven to 350°F (175°C). Grease and flour a 9-inch square baking pan.
Whisk together the flour and baking soda. Set aside.
Using an electric mixer on medium speed, beat the butter and 1 & 1/4 cups (250g) sugar until light and fluffy. Add the egg yolks, one at a time, beating just until combined after each addition.
Reduce mixer speed to low. Add the flour mixture in three portions, alternating with two portions of yogurt. Stir in the lemon zest.
In a separate bowl, beat the egg whites with a mixer at high speed until foamy. Gradually add 1/4 cup (50g) sugar and continue beating until stiff peaks form. Gently fold into the batter.
Transfer the batter to the prepared pan. Drop the lemon curd over the batter. Use a knife to swirl the curd gently into the cake. Sprinkle with the chilled crumb topping.
Bake 45 to 50 minutes, our until a pick inserted into the center comes out clean.