Grease and line a 9"x9" square cake tin with baking paper, leaving an overhang for easy removal of the slice. Preheat the oven to 170°C/340°F.
In a large mixing bowl, combine the butter, sugar and vanilla. Cream until the butter has lightened in colour (for about 2 minutes).
225 grams butter, 150 grams white granulated sugar, 1 teaspoon vanilla extract
Weigh out the flour, and add the baking powder and salt. Whisk together to evenly distribute the baking powder and salt through the flour.
330 grams plain flour, ½ teaspoon table salt, 1 teaspoon baking powder
Place half the flour into the butter mixture and mix through until just combined. Then add in the other half and mix until a soft pliable dough forms. Knead the dough a few times in the bowl until the dough comes together.
Press ⅔ of the dough into the bottom of the lined cake tin until it is evenly and smoothly pressed into the tin. Evenly spread the jam over the layer of biscuit dough. Then crumble the last ⅓ of the biscuit dough over the top of the jam.
⅓ - ⅔ cup jam
Bake the jam slice for 35 to 40 minutes or until the biscuit dough is beautifully golden brown. Remove from the oven and allow the squares to cool completely in the tin. When the jam squares are cool, remove them from the tin and slice them into 16 to 25 squares or bars.