Preheat the oven to 425f and grease and line a 13 x 9 inch sheet pan.
In a large bowl, mix together the flour and sugar then add in the diced cold better and using a pastry blender, work butter into the flour mixture until it is crumbly.
Add in the one and a quarter cups of cold buttermilk and three teaspoons of the vanilla extract and stir until combined.
Turn out into the prepared tin and press down into an even layer using lightly floured hands.
Brush with the tablespoon of buttermilk and sprinkle over a tablespoon of sugar.
Bake in the oven for 16-18 minutes until the top is starting to turn golden brown then allow to cool completely in the pan.
Once cooled, place the strawberries and one teaspoon of vanilla extract into a bowl and stir. Allow to stand for fifteen minutes stirring occasionally.
Place the heavy whipping cream, three tablespoons of sugar and one teaspoon of vanilla extract into a bowl and whisk until soft peaks form.
Heat the strawberry jam over a low heat until loose and smooth.
Remove the shortcake from the pan and place onto a large platter.
Spread over the whipped cream, top with the strawberries (you can be creative with your design if you wish) and drizzle over the strawberry jam.
Serve immediately.