Preheat oven to 350F/180C. Grease 2 8-inch cake pans and line the bottom with parchment rounds--grease the parchment too.
Cream butter and sugar until light and fluffy; about 4 minutes.
Add the eggs, beating well after each addition, then add the vanilla.
Fold the flour into the butter mixture until you cannot see any flour particles.
Divide the mixture equally between the pans--weighing is the best way if you want it to look even and perfect.
Bake for 20-25 minutes or until a cake tester comes out clean. Cool completely--you can safely cool the cakes in the pans.
To assemble, unmold the cakes from the pans, carefully peel off the parchment and level off the top of at least 1 layer so it will be flat on the serving plate. Likewise you can level off the top of the other layer if you want a flat top. If you don't mind a domed top, that isn't necessary.
Beat the cream until stiff peaks form (see post body on how I stabilize the cream). Spread the jan on top of the bottom layer then spread the whipped cream over that. Top with the other layer (leveled side on top of the cream if you want the flat top) and dust with confectioner's sugar if desired.