Prep and preheat: Heat the oven to 350Β°F (175Β°C).
Line an 8-inch round cake pan with parchment and grease the sides. For clean layers, use a springform pan if you have one.
Mix dry ingredients: In a bowl, whisk flour, baking powder, and salt. Set aside.
Cream butter and sugar: In a large bowl, beat butter and sugar until pale and fluffy, about 2β3 minutes.
Add eggs one at a time, mixing well after each. Stir in vanilla.
Add dry and milk: Mix in half the dry ingredients, then all the milk, then the remaining dry ingredients. Stir just until combined.
Donβt overmix.
Bake the cake: Pour into the pan and smooth the top. Bake 22β28 minutes, until a toothpick comes out clean. Cool 10 minutes in the pan, then turn out onto a rack and cool completely.
Make the custard: In a saucepan, heat milk until steaming.
In a bowl, whisk sugar, egg yolks, and cornstarch until smooth. Slowly whisk hot milk into the yolk mixture. Return everything to the saucepan and cook over medium heat, whisking constantly, until thick and bubbling, 2β3 minutes.
Finish the custard: Remove from heat.
Whisk in vanilla and butter until glossy. If using strawberry jam, stir it in now for a gentle berry note. Press plastic wrap directly onto the surface and chill until cold and set, about 1β2 hours.
Prep the strawberries: Toss sliced berries with sugar and lemon juice.
Let sit 10β15 minutes to release juices.
Assemble: Place the cooled cake on a serving plate or the base of your springform. Spread the chilled custard evenly over the top, leaving a small border if the custard is very soft. Arrange strawberries on top.
If using, brush with warmed jelly for shine.
Chill and serve: Chill at least 1 hour to help the custard set and the flavors meld. Add a ring of whipped cream just before serving if you like. Slice with a sharp knife, wiping between cuts.