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

Add 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, around 8-10 minutes.

Add the cornstarch slurry into the pan and stir to combine. Cook until the mixture thickens and sticks to the back of a spoon. Remove from heat.

Allow the mixture to cool completely, then transfer to an airtight container or bowl covered with plastic wrap and refrigerate for at least 1 hour before using.

Preheat the oven to 350°F and prepare two 8 inch round cake pans. Spray with cooking spray and line with parchment paper.

In a large bowl, whisk flour, cocoa powder, sugar, salt, baking powder, and baking soda together.

Add eggs, vanilla, oil, and milk. Pour in hot water and mix until smooth.

Divide the batter evenly between the two cake pans and bake for 20-26 minutes.

Let the cakes cool in the pans for 20 minutes, then loosen and flip onto a cooling rack to cool completely.

While the cakes cool, make the buttercream.

Add softened butter to a large bowl. Slowly add powdered sugar with a mixer until combined.

Add ¼ cup of the strawberry cake filling to the buttercream and mix until fully combined. Adjust consistency with more powdered sugar if needed.

Cover and refrigerate the buttercream until the cakes are cool.

Once the cake layers and filling have cooled completely, place one cake layer on your cake plate and pipe a thick layer of frosting around the edges.

Scoop the strawberry filling onto the cake, creating a circle with frosting around it. Place the second cake layer on top.

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