Preheat oven to 320 degrees F convection. Line bottom of an 8in cake pan with parchment paper and brush melted butter on the paper.
Cream together the butter and sugar until pale and fluffy. Beat in two whole eggs one at a time, plus two yolks only.
Mix in the vegetable oil. Then add the lemon extract.
Mix in the flour and pour the batter into the prepared tin.
Bake for 40-45 minutes. Test after 35 minutes by shaking the pan slightly and see the center is solid, and the edges are starting to pull away from the pan. Leave to cool before adding the curd.
Grape Curd and Lemon glaze
Make the Grape curd while the cake is baking, or ahead of time, as the curd needs to chill!
Whisk together the grape juice, sugar, egg yolks & cornstarch, in a saucepan.
Heat gently, stirring continuously, until it suddenly thickens. Don't heat more than 30 seconds beyond the thickening.
Remove from heat and stir in the butter. Immediately pour through a strainer to remove any egg solids. Then put in the fridge to cool.
When the cake is cooled, slice in half horizontally. Spoon the chilled grape curd on the lower half. I think a bit of curd spilling out adds good color! Place the upper half on top!
Prepare the lemon glaze by combining the lemon juice and last sugar ingredient in a saucepan. Heat gently to boiling, stir regularly, continuing to boil.
As it boils it will slightly thicken. When the spoon can pour and show it is no longer runny, remove from heat and immediately spoon over the cake. The mixture will solidify quickly so don't hesitate. Don't try to spread, just pour from the spoon!