Prep the pan and oven: Heat the oven to 350°F (175°C).
Line the bottom of an 8-inch or 9-inch round cake pan with parchment. Do not grease the sides; it helps the sponge rise.
Whip the eggs and sugar: In a large bowl, beat the eggs, sugar, and vanilla on high speed for 6–8 minutes until very pale, thick, and tripled in volume. The mixture should leave ribbons when you lift the beaters.
Fold in dry ingredients: Sift together the flour, baking powder, and salt.
Gently fold into the egg mixture in 2–3 additions using a spatula. Avoid deflating the batter.
Add milk and oil: In a small bowl, mix the milk and oil. Stir a spoonful of batter into this mixture, then fold it back into the main batter.
This keeps the texture light.
Bake: Pour the batter into the pan and smooth the top. Bake 20–25 minutes, or until the top is golden and a toothpick comes out clean. Cool 10 minutes in the pan, then run a knife around the edges and turn out onto a rack to cool completely.
Make the syrup: Heat water and sugar in a small pan until the sugar dissolves.
Remove from heat and add lemon juice or vanilla if using. Cool completely.
Prep the fruit: Slice or chop fruit into bite-size pieces. If using berries, pat them dry.
Toss gently with lemon juice if desired.
Whip the cream: Beat cold heavy cream, powdered sugar, vanilla, and a pinch of salt on medium-high until medium-stiff peaks form. Do not overbeat.
Slice the cake: With a serrated knife, carefully slice the fully cooled cake into two even layers.
Assemble the cake: Place the bottom layer on a serving plate. Brush with syrup until lightly moistened.
Spread a generous layer of whipped cream, then scatter an even layer of fruit. Top with the second cake layer and brush with more syrup.
Finish and chill: Spread the remaining whipped cream over the top and sides. Decorate with more fruit on top.
Chill at least 1 hour to set and let the flavors meld.
Serve: Slice with a sharp knife, wiping the blade between cuts for clean slices.