Streusel:
½ cup flour
½ cup packed light brown sugar
¼ tsp salt
4 Tbsp butter, melted
½ cup sliced almonds
Cake:
6 Tbsp butter
1 entire orange, scrubbed and cut into large chunks and seeded (yes, rind and all!)
¼ cup sour cream, at room temperature
1½ cups flour
1 tsp baking powder
½ tsp salt
¼ tsp baking soda
¾ cup sugar
2 large eggs, at room temperature
Glaze (optional):
3 to 4 tsp freshly squeezed orange juice (from 1 orange)
¾ cup icing sugar
Method:
Preheat the oven to 180 degrees Celsius.
Prepare the streusel: In a medium bowl, combine the flour, brown sugar, and salt. Drizzle the melted butter over the mixture and stir to incorporate. The mixture should clump together when squeezed. Mix in the almonds.
Line an 8-inch square baking pan with baking paper.
Place the orange in a blender and process. It’s okay if you have a few larger pieces. You should have about 1 cup of orange purée. Add the sour cream and stir to combine.
In a medium bowl, whisk together the flour, baking powder, salt, and baking soda. In a large bowl, with an electric mixer on medium speed, beat the butter and granulated sugar until light and fluffy, about 3 minutes. Add the eggs, 1 at a time, scraping down the bowl as necessary. Add half of the flour mixture and beat on low speed until just combined. Beat in the orange mixture, then beat in the remaining half of the flour mixture.
Transfer the batter to the prepared pan and smooth the top. Top with the streusel mixture. Squeeze the streusel to form a range of differently sized clumps. Bake until a toothpick inserted into the center of the cake comes out with moist crumbs attached, 40 to 45 minutes. Transfer the pan to a wire rack to cool for 20 minutes. Then, transfer the cake to a wire rack to cool completely.
Prepare the glaze (if using): In a small bowl, whisk the orange juice into the confectioners’ sugar, adding a little less juice for a thicker glaze that will look lovely on top of the cake, or a little more for a thinner glaze that will soak in. Drizzle the glaze over the cooled cake.