Preheat oven to 350°F. Grease two 6-inch cake pans and line with parchment, if you like.
Cream together sugar and butter until fluffy. Beat in eggs one at a time, then add vanilla.
Slowly mix in flour until well incorporated.
Divide batter between cake pans and bake for 20-25 minutes, until a toothpick inserted in the center comes out clean.
Let cakes cool in pans 10 minutes, then turn out onto racks to cool completely.
Custard
In a medium saucepan, heat milk and ginger just until bubbles begin to form at the edges.
In a bowl, whisk together eggs, cornstarch, sugar, and salt. Set over a larger bowl filled with ice cubes.
Slowly pour the heated milk into the eggs, whisking continuously (to prevent the eggs from scrambling).
Pour the mixture back into the saucepan and heat over medium-low, stirring constantly, until mixture has thickened (about 5 minutes).
Strain custard through a fine mesh sieve into a clean bowl.
Cover custard with plastic wrap, pressing down onto the surface to prevent a skin from forming, and chill for at least 2 hours.
To Finish
Level cake layers, if needed. Set bottom layer on a cake plate. Spread 1/3 cup of preserves on top. Gently spoon thickened custard on top of the jam, leaving an edge all the way around the cake to prevent weeping.
Spread the rest of the jam on the second layer, then set it on top of the first layer, jam-side-up. Decorate with edible flowers, if you like.