Line an 8” square cake pan or a 7” round springform pan with parchment paper. Lightly butter the sides of the pan and the parchment. (The springform makes a slightly taller cake.)
Prepare the crumb first; refrigerate while you make the cake batter.
For the crumb:
1/3 cup granulated sugar
1 teaspoon orange zest
1/3 cup brown sugar
1 teaspoon cinnamon
1/8 teaspoon salt
8 tablespoons butter, melted
1¾ cups cake flour, sifted
Rub the orange zest into the granulated sugar with your fingers. Add the brown sugar, cinnamon and salt. Whisk together then add the melted butter. Add the cake flour last, (I use my fingers) and pinch the mixture; you want crumbs about 1/2 –inch in size. Cover and refrigerate. Turn the oven on to 325 degrees F.
For the cake:
1 cup cake flour, sifted
½ teaspoon baking soda
½ teaspoon baking powder
¼ teaspoon salt
½ cup granulated sugar
1 tablespoon orange zest
6 tablespoons unsalted butter, room temp.
1 large egg
1 large egg yolk
1/3 cup sour cream or whole fat Greek yogurt
2 tablespoon fresh orange juice*
1½ teaspoons vanilla
(for more concentrated orange flavor, start with ¼ cup fresh orange juice and reduce it down to 2 tablespoons- I do this on top of the stove then let it cool.)
In a medium bowl, whisk together the flour, baking soda, baking powder and salt.
In a small bowl, combine the granulated sugar with the orange zest, rubbing it together with your fingers. Use a standing mixer fitted with the paddle attachment (or a handheld mixer and a large bowl) beat the butter on medium speed, gradually adding the orange zest/sugar, until the mixture is smooth. Using the same bowl you used for the sugar and zest, whisk together the egg, egg yolk, orange juice and vanilla. Gradually add the egg mixture to the butter mixture, scraping down the sides and bottom of the bowl. On low speed, add half of the flour mixture, beating briefly until smooth, then beat in the sour cream, scraping down the bottom and sides of the bowl. Add the rest of the flour mixture, beating on low just until combined; do not over beat. Scrape the batter into the pan, spreading it evenly using a small offset spatula. Retrieve the crumb mixture from the fridge, break it up into large clumps and sprinkle evenly over the batter. Place the cake on the middle rack of the oven. Baking time varies by oven; I start checking the cake at 50 minutes-it should test clean. If not, give it another 5-10 minutes. Baking at a low temperature keeps the cake moist and the crumbs from getting too dark, but it can take as long as 60 minutes in the oven so keep an eye on things- then transfer the cake to a rack to cool completely before slicing. Cover and wrap any leftovers; it can sit at room temperature for up to 3 days.