FOR THE STRAWBERRY SHEET CAKE: Place the strawberries in a blender and blend to a puree. Strain them through a sieve into a medium saucepan.
Heat the puree on medium heat and let it simmer for 15-20 minutes, stirring often until it has reduced right down to half a cup. Transfer to a small bowl and chill just until it reaches room temperature or just over (notes).
Preheat the oven to 180C / 350F / 160C fan forced. Grease and line a 9x13 tin with baking paper. If your tin has short sides, make sure the paper comes up past the top edge by an inch or so.
In a medium mixing bowl, whisk together the flour, cornflour, baking powder, baking soda and salt to lighten and evenly disperse. Set aside.
Using a large bowl and handheld electric beater or stand mixer with paddle attachment, beat together the butter, oil and sugar until pale and creamy - about 2 minutes.
Add the eggs, one at a time, beating each until the mixture is smooth. Scrape down the sides of the bowl from time to time.
In a jug, mix together the strawberry puree, buttermilk and vanilla.
Add ⅓ of the flour mixture to the butter and egg mix. Beat on low or stir until just combined. Follow with half the buttermilk mixture, mixing until just combined.
Now, using a spatula, not the mixer, repeat the process using three more instalments - flour, buttermilk, flour - mixing each time until just combined. If using colouring, add a couple of drops with the last amount of buttermilk.
Pour the batter into the prepared baking tin and gently spread out evenly all over. If you make the centre slightly lower than the edges, you'll end up with a flatter cake surface as the centre does rise.
Bake for around 28 minutes. Start checking 3-4 minutes before by poking with a toothpick - it's done when the toothpick comes out with just a crumb or two attached.
FOR THE STRAWBERRY ERMINE FROSTING: Whisk the flour, milk and half the sugar together in a small saucepan until smooth. Heat over a low-medium heat, continuing to whisk slowly so that it doesn't form lumps, until it gets very thick (like a pudding or thick custard).
Transfer the mixture to a bowl and cover with plastic wrap pressed to the surface. Cool this mix to room temperature (notes 7).
Beat together the butter and remaining ½ cup sugar until really creamy - about 3-4 minutes. Scrape down the bowl a few times through the process.
Add the cooled flour paste a large spoonful at a time, beating on medium until each spoonful is incorporated before adding the next.
Once the paste has all been added, mix on medium for another 5-6 minutes. If it looks like it separates, don't worry, just keep on beating. It will all come together into a beautiful, almost whipped cream looking frosting.
Once the frosting looks whipped and light (and not split), add the freeze-dried strawberry powder, vanilla and salt. Beat on low just to incorporate then stop.
Optional: To remove some of the air-bubbles from the frosting and make it more smooth, use a spatula to press it up against the sides of the bowl a few times.
Top the cooled cake with the frosting, then decorate with extra strawberries.
I use a standard Australian 20ml tablespoon (= 4 teaspoons worldwide)
For best results, you should always weigh ingredients like flour and sugar. Kitchen scales are relatively cheap but if you can’t weigh the ingredients, use the spoon and level method (don’t scoop).
All ovens vary - check your cake for doneness 3-4 minutes before the recipe suggests.
You can swap both the plain flour and cornflour for cake flour.
The puree must be cooled to around room. If your buttermilk is still quite cold, then it's ok if the puree is still a touch warm as they'll even each other out when you combine them.
You can make this cake without reducing the strawberry puree but you won't be able to taste strawberry in the cake. If you want to skip the reduction, you only need ⅓ cup of non-reduced strained strawberry puree.
It's important to cool the flour and milk paste to room temperature before adding to your butter. While you can cool it more quickly in the fridge or freezer, it must be room temperature before using it - neither hot nor cold.