In the bowl of a food processor, combine 1 ½ cups all-purpose flour, ⅔ cup sugar, 1 tablespoon lemon zest, 1 tablespoon orange zest, and an ⅛ teaspoon kosher salt. Pulse to combine and then add ½ cup very cold butter, diced. Pulse until crumbly. Refrigerate until ready to use. (See notes section)

Preheat the oven to 350°F. Grease and flour an 9" square pan.

Mix wet ingredients - Using a mixer with the beater attachment, cream ½ cup butter. Add 1 ¼ cups sugar and continue beating until light and fluffy. Add egg yolks one at a time, beating after each addition to combine.

Mix dry ingredients - In a separate bowl, combine 1 ½ cups all-purpose flour and ⅛ teaspoon baking soda.

Add flour mixture to wet ingredients alternately with ½ cup plain Greek yogurt, beginning and ending with flour mixture.

Stir in 1 tablespoon lemon zest.

In a separate bowl, using the whisk attachment, whisk 3 egg whites at high speed until soft peaks form, about 1-2 minutes (see notes section).

Gradually add remaining ¼ cup sugar a little at a time, beating another minute or so until glossy, stiff peaks form (see notes section).

Gently fold egg whites into batter and pour batter into a greased and floured 9-inch square (2-inch-deep) pan.

Dollop with lemon curd and gently swirl with a knife. Sprinkle Crumb Topping over batter (see notes section).

Bake at 350°F for 55 to 60 minutes (see notes section)

Remove from oven and dust with powdered sugar (see notes section).

Let the cake cool completely before cutting (see notes section)