Preheat your oven to 170C / 340F (fan off), 150C / 300F (fan on). Position the rack in the middle of the oven. Spray the bottom and sides of a 22cm / 9-inch square baking tin with oil spray. Line with the bottom with baking paper.
Combine dry ingredients - To a medium sized mixing bowl, add the flour, baking powder and salt. Use a whisk to combine. Set aside.
Beat the eggs - Add the eggs to a large mixing bowl and use an electric mixer to beat until frothy. This can also be done in a stand mixer with a whisk attachment. While the mixer is on high speed begin slowly adding the sugar. This should take about a minute. Once it’s all in there, continue beating on high speed for about 6 minutes until it’s pale and thick and tripled in size.
Hot milk and butter - Add the milk and butter to a saucepan and heat on medium heat, stirring continuously until small bubbles begin to form on the sides of the pot. Do not let the butter form a skin as it will not dissolve in the cake batter.
Add dry ingredients - Add the dry ingredients to the eggs and use a spatula to fold through until no dry ingredients are showing taking care to scrape from the bottom where the dry ingredients like to hide. The batter will be quite thick.
Add hot milk mixture - Add 1/2 of the hot milk mixture and all of the oil and vanilla extract. Use the spatula or whisk to gently fold through. Add the remaining milk mixture and fold through until the mixture is well combined and even in texture. It will be thin, with a little volume and fall out of the bowl into the cake pans in a thin ribbon.
To bake - Add mixture to the cake tin. Tap gently on your workbench 3 times to release air bubbles before baking on the middle rack of your oven for 30 minutes or until a toothpick inserted into the centre comes out clean. Once baked, allow to cool in the tin for 10 minutes before carefully running a knife around the sides to release it. Transfer to a cooling rack to cool completely to room temperature.
To trim and fill the cake - This part is optional. Use a large serrated knife or cake leveller to trim the cooled cake in half. Add the raspberry jam to one half and spread as evenly as you can. Then sandwich with the other cake half. Wrap in plastic and place in the fridge to chill for one hour.