Preparation

Make the streusel

• In a small bowl, stir the flour, brown sugar, cinnamon, and salt. Drizzle the melted butter over the dry ingredients and stir until well combined. The streusel should feel clumpy, not sandy, when gently squeezed between your fingertips. If it seems dry, add more melted butter.

Make the cake

• Position a rack in the center of the oven and heat the oven to 350°F. Lightly butter the bottom and sides of a 8-inch square metal cake pan. Line the bottom of the pan with parchment and butter the parchment. Dust the pan with flour, tapping out any excess.

  In a small bowl, whisk the sour cream, vanilla, and almond extract.

  In a large bowl, cream the butter, granulated sugar, and egg yolk with a wooden spoon until blended, about 20 seconds. Using a whisk, whisk in the whole egg and salt and continue to whisk until the batter is smooth and the sugar begins to dissolve, about 30 seconds. Whisk in the sour cream mixture. Sift the cake flour, baking powder, and baking soda directly onto the batter. Whisk until the mixture is smooth and free of lumps.

  Spread the batter evenly in the pan. Sprinkle the streusel over the batter, squeezing it with your fingertips to form small clumps. Bake until puffed and golden and a skewer inserted in the center of the cake comes out with only moist crumbs clinging to it, 28 to 30 minutes. Set the pan on a rack to cool for 15 minutes.

  Run a knife between the cake and the sides of the pan. Invert the cake onto the rack and remove the parchment. Invert again onto a serving plate, so the streusel is on top. Let cool at least 10 minutes before serving. This cake is best served warm.