Preheat the oven to 180C/350F and grease and line two 8 inch round tins. I spray my tins with Bake Easy and then line the bottom and sides with grease proof paper.
Make the cake: Place butter and sugar into a large bowl, or the bowl of your stand mixer and beat until light and fluffy, around 2 minutes on medium-high speed.
Add in the eggs and vanilla extract, scraping down the sides as necessary, and mix on medium speed until well incorporated and smooth.
Add in the flour, baking powder and salt and mix on low until the mixture begins to come together, then add the milk and mix on medium speed until everything is well combined and there are no lumps of flour.
Divide the batter between the two cake tins; I use a half cup measure to divide my batter evenly.
Place in the oven for 22-27 minutes, or until risen, golden on the top and an inserted skewer into the centre comes out clean. Leave to cool in the pans for at least 45 minutes before transferring to a wire rack to cool completely.
Whilst the cakes are cooling, make the jam: Place the berries and sugar into a medium sized saucepan over a medium heat. Bring to a rolling boil, stirring frequently to ensure the sugar doesn't burn. Continue to boil for 5 minutes and then add the 2tbsp of Certo. Continue to cook, stirring constantly, for 7-10 minutes until the mixture thickens and coats the back of your spoon.
You will reach a point where you feel it thicken; it will thicken more as it cools, but you will feel it become gloopy. - Another way to tell is the mixture will pull away from the pan when it's ready; as in when you're stirring it round the pan, you will see the bottom of the pan where you're stirring as it pulls away. Leave to cool completely.
Once the jam and cakes are cool, make the frosting: Place butter, sugar, salt and vanilla into a large bowl and beat on low until the mixture starts to come together, then turn the speed to high and continue to beat until the colour gets paler and your mixture is light and fluffy. Add in the milk 1/2tbsp at a time, if required.
Place a small amount of frosting on a plate/cake board. Place one of the cakes bottom side down into the centre of the plate/board.
Spoon the jam into the centre of the cake and spread into a smooth, even layer with a small angled spatula. You may not need it all, in which case save the extra jam and have it on toast!
Spoon enough frosting into the centre of the cake to cover it in a thin layer, around 1/8 inch thick.
Place the second cake layer, bottom side up (the bottom is the flattest and will result in a better finish, this is why I always put the cake "upside down!").
Using either a large or small angled spatula, spoon frosting into the centre of the cake and then smooth over the top and down the sides. I use a small spatula as I like the control it gives me.
The top doesn't need to look too perfect, just smooth it as best you can. For the sides, I place the cake on an icing turntable and then spin the cake slowly as I smooth the frosting over. To get a really smooth finish, dip the spatula in warm water, then lightly run it over the frosting.
Top the cake with berries; I started with strawberries in the middle, then placed raspberries and blueberries around the edge and piled blackberries and a few more strawberries on the top in the middle to give some height!
Summer Berry Layer Cake will keep covered, in the fridge for 3 days. Leave to warm up for 30 minutes before serving unless you like a cold cake! (I don't mind, but I know some people don't like it!)