Add the raspberries and sugar to a small saucepan, and bring to a gentle simmer. Cook for 10-15 minutes, stirring often, until the berries have broken down and the sauce has slightly thickened. Sift the cornstarch into the pan, and stir well. Continue to cook for just about another minute, then remove from the heat. Push the cooked raspberries through a fine mesh strainer to remove the seeds. You'll want to use a flexible silicone spatula or the back of a spoon to press the sauce through the strainer so that you get as much of the berries through as possible.
Set aside to cool completely. This can be done up to a couple of days in advance and kept in the fridge until ready to use.
For the cake
Preheat oven to 350F and grease 2 8-inch cake pans. I also line the bottom of mine with a circle of parchment paper for easy removal.
Melt the butter in a small saucepan or the microwave and allow it to cool.
In a large mixing bowl, add the oil, sugar, eggs, vanilla extract, almond extract, milk and melted butter and whisk everything together until smooth.
In a separate bowl, whisk together the flour, baking soda, baking powder and salt, and add it to the wet ingredients. Mix to combine but don't over mix.
Finally, add the hot water along with the vinegar, and fold the batter together until all the ingredients are well mixed. The batter will be thin, and you may even see it start to fizz a little bit from the vinegar reacting with the baking soda. This is ok!
Divide the batter evenly between the two prepared pans and bake for 30-35 minutes until risen and golden, and a toothpick inserted into the center of the cake comes out without wet batter clinging to it.
Let the cakes cool for 15 minutes, then run a blunt knife around the edges of the pans and flip them over to a cooling rack to finish cooling completely before frosting.
For the raspberry buttercream
Cream the butter for a minute or two in a stand mixer with a paddle attachment (or with electric beaters) until smooth.
Add the sifted confectioner's sugar, about a cup at a time, mixing well between each addition.
Add the raspberry filling, cream, vanilla extract, and salt. Feel free to add a little more or less of the liquid ingredients depending on your preference for taste and texture (leave about 1/4 cup or so of raspberry filling to add to the middle of your cake.) Beat the frosting until smooth and creamy.
To frost the cake, add a layer of raspberry buttercream to the first cake layer, and top with a thin layer of the remaining raspberry filling. Add the next cake layer on top, and use the remaining frosting to cover the top and sides. Top with fresh raspberries, if desired.
Notes
Make ahead tips:
The raspberry sauce can be made up to a few days in advance and refrigerated.
The cake layers can be made ahead, cooled completely, wrapped in plastic and kept overnight on the counter, or in the refrigerator.
The frosting can be made up to a day ahead, covered with plastic in the refrigerator. Bring to room temp and re-whip before using.
Pan size conversions:
This cake can be baked in a 9x13 pan, same oven temperature, and increase the baking time a bit.
You can halve the recipe for a single layer cake.
If you want to use 9 inch cake pans decrease the baking time a bit.
You can make cupcakes, same oven temp, check them at 18-20 minutes.