In a small bowl, whisk cornstarch and water together until combined and set aside.

Add chopped strawberries, sugar, and lemon juice to a medium or large pan on medium low heat.

Stir to combine and continue stirring every few minutes until the mixture reaches a simmer. Reduce the heat to low and cook until the mixture has reduced. This should take around 8-10 minutes.

Add the cornstarch slurry into the pan and stir to combine. The mixture will continue to thicken for a few more minutes. Remove from heat when the mixture sticks to the back of a spoon.

Set aside and allow the mixture to cool completely. It will continue to thicken as it cools.

Once cooled, transfer the filling to an airtight container or a bowl tightly covered with plastic wrap and refrigerate for at least one hour before using.

Preheat the oven to 350°F and prepare two 8 inch round cake pans. Spray the inside with cooking spray, then cut out pieces of parchment paper so that they fit in the bottom of each pan.

Using a hand mixer or stand mixer with the paddle attachment, beat the butter and oil together until combined. Add in the eggs and vanilla extract.

Mix in the granulated sugar.

Add in flour, cornstarch, salt, and baking powder to a separate bowl and whisk together.

Alternate slowly adding in the milk and the dry ingredients. Mix until combined.

Divide the cake batter between the two cake pans and bake for 24-30 minutes, or until a toothpick inserted in the middle of the cakes comes out clean.

Let the cakes cool in the pan for 10-15 minutes, then remove them from the pan.

Add the butter to a large bowl. Slowly add in powdered sugar, using a standing mixer or handheld mixer to mix until combined.

Add ¼ cup of the strawberry cake filling to the buttercream and mix until fully combined. If needed, slowly add in more powdered sugar until it reaches your desired consistency.

Cover and refrigerate until the cakes have cooled completely.

Once the cake layers and filling have cooled completely, assemble the cake. Place one cake layer onto your cake plate and pipe a thick layer of frosting on top of the cake.

Scoop the strawberry filling onto the cake, so that there is a circle of buttercream around it. Place the second cake layer on top.

Frost the cake and decorate as desired. Remaining buttercream can be frozen.