Preheat the oven to 350° F. Butter a 9 x 13 inch cake pan and set aside.: As the oven warms you will notice a faint warm, toasty smell; this gentle heat sets the stage for even baking. Buttering the pan well ensures the cake releases cleanly, and I sometimes line the bottom with parchment for extra insurance. If your oven runs hot, rotate the pan halfway through baking to prevent uneven browning, and avoid opening the oven door too often because that lets out heat and can deflate the sponge.
In a medium mixing bowl add the flour, baking powder, and salt and whisk to combine. Set aside.: You should see a uniform pale mixture with no lumps, and whisking traps a little air which helps the batter stay light. This dry mix distributes leavening so the cake rises evenly; failing to combine can leave pockets of baking powder, which produce odd bumps. Sift if your flour is clumpy, but do not overwork the mixture once liquids are added.
In a large mixing bowl or bowl of a stand mixer, add the eggs and sugar. Beat on medium speed until pale and fluffy, about 10 minutes. Add in the vanilla and mix just until combined.: Early on you will hear a steady, airy whipping sound and watch the eggs transform into a ribbon like batter, light in color and thick enough to form trails. This aeration is essential, it creates the sponge's lift and the structure needed to absorb the milks. A common mistake is underbeating, which yields a dense cake, so be patient and stop when the batter looks glossy and holds soft peaks.
Add half of the flour mixture to the egg mixture and beat just until combined. Mix in the whole milk until combined and then mix in the rest of the flour mixture until just combined.: At this stage you want to fold or beat briefly to preserve air bubbles; the texture should become smooth but still buoyant. The milk loosens the batter, making it silkier, and the staged addition prevents overmixing. Overdoing the mixing will develop gluten and make the final cake less tender, so aim for gentle, efficient motions.
Pour the batter into prepared cake pan and bake for 25-30 minutes or until cake springs back when lightly touched. Allow to cool for 45 minutes.: As it bakes you will see the top turn faintly golden and smell a warm, comforting aroma. The spring back test and a toothpick with a few moist crumbs are your cues that it is done; underbaking leaves a gummy center that will resist the soak. Letting it cool slightly is crucial, because pouring the cool soak onto a hot cake causes it to absorb unevenly.
While the cake is cooling prepare the filling. Add the sweetened condensed milk, evaporated milk, heavy cream, ground cinnamon, and nutmeg to a medium mixing bowl and whisk until well-combined.: Whisking blends the milks into a silky, slightly viscous soak that smells faintly sweet and spiced. The texture should be smooth, with no streaks of condensed milk, and the spices should be evenly distributed. If you skip thorough mixing you might get clumps of condensed milk that settle, leading to pockets of intense sweetness when poured.
When the cake has cooled use a fork to pierce the entire surface of the cake. Gently pour the filling over the cake.: Piercing creates channels for the milks to sink deep into the crumb, and as you pour you will see the cake accept the liquid slowly, almost like a sponge peacefully drinking. Pouring too quickly floods the top and leaves the interior dry, so move slowly and allow the soak to be absorbed before adding more. If the cake resists liquid, use additional fork pricks in denser spots.
Add the heavy cream and sugar to the bowl of a stand mixer and beat on high speed for 7-10 minutes, or until fluffy and somewhat stiff. Add in the clear vanilla extract and gently combine with a rubber spatula. Spread the topping over the cake.: You will hear the mixer speed change as the cream thickens and watch soft peaks become a stable cloud like topping. The whipped cream should be cool and billowy, with a silky sheen, and folding in the vanilla at the end preserves its volume. Overwhipping turns the cream grainy or into butter, so stop as soon as it holds shape but is still soft enough to spread.
Cover and chill the cake in the refrigerator for 2 hours or overnight before slicing and serving. Top with fresh berries and/or a sprinkle of ground cinnamon right before serving.: Chilling lets the soak set and the flavors meld, giving a consistently moistened crumb and a firm slice. Overnight chilling deepens the flavor and improves texture, and the cake will taste cooler and more refreshing. A common error is slicing too soon, which leads to a sloppy cut and milks running out, so be patient for clean slices and add garnishes just before serving to keep them bright.