Preheat the oven to 350°F.: The warmth fills the kitchen with a faint scent of metal and heat, and you should feel the oven stabilize before you place the pans inside. A properly preheated oven ensures even rise and predictable browning on the cakes surface. If you skip this, cakes can bake unevenly or take longer, so allow the oven to reach temperature and avoid opening the door repeatedly while baking.
Butter the bottom of two 8-inch cake pans. Then line them with parchment paper and butter and flour the lined pans.: Greasing and lining introduces a tactile routine, where you press the parchment into the pan and feel the give of the paper. The butter creates a slick surface so your cakes release easily, and a thin dusting of flour prevents sticking. If you skip parchment, cakes can tear when you remove them, so take these small steps to ensure a clean release and even browning.
Cream the butter and sugar on high speed in the bowl of an electric mixer fitted with the paddle attachment until light and fluffy. On medium speed, add the eggs, 1 at a time, then the sour cream, zests, and vanilla, scraping down the bowl as needed. Mix well. Sift together the flour, cornstarch, salt, and baking soda. On low speed, slowly add the flour mixture to the butter mixture and combine just until smooth.: As you whip the butter and granulated sugar , watch the texture change from dense to pale and airy, and listen for the smoother, quieter whir of an aerated batter. Adding eggs one at a time helps maintain that emulsion, preventing separation. The sour cream and zests bring moistness and aromatics. Combining dry ingredients by sifting ensures even distribution of baking soda and prevents lumps of cornstarch . When adding dry mix on low, you preserve the trapped air, which is crucial for a tender crumb. A common mistake here is overmixing after the flour goes in, which can produce a dense cake, so stop once the batter looks homogeneous and smooth.
Pour the batter evenly into the pans, smooth the tops, and bake in the center of the oven for 40 to 45 minutes, until a toothpick comes out clean. Let cool in the pans for 30 minutes, then remove to wire racks and let cool to room temperature. If using 1 cake, wrap the second well and freeze.: As you pour, notice the batter flowing thickly and evenly, then smooth the surface with a spatula. While baking, the kitchen fills with a gentle, sweet aroma that hints at butter and citrus. The tops should turn lightly golden and spring back when touched. The toothpick test ensures the center is set without overbaking, which would dry the crumb. Cooling in the pan helps the cake settle, so resist removing too soon and avoid slicing warm layers, which can compress and tear.
To make the filling for one cake, whip the cream, sugar, and vanilla in a mixer fitted with the whisk attachment until firm. Slice one of the cakes in half with a long, sharp knife. Place the bottom slice of the cake on a serving platter, spread with 1/2 the whipped cream and scatter with sliced strawberries. Cover with the top slice of the cake and spread with the remaining cream. Decorate with strawberries.: When whipping the heavy cream , the sound changes from splashing to a heavier patter as it thickens, and visually it goes from glossy to billowy peaks. Using a long, sharp knife to slice a cake yields clean edges, and take your time for an even cut. Spreading the cream should feel cool and smooth against the cake, and the sliced strawberries add bursts of ruby color and juicy texture. Avoid overwhipping the cream to dry stiffness, which makes it lose silkiness, and if your berries are very wet, blot them so the filling does not become soggy.