Make the filling: In a large saucepan, heat condensed milk, butter, and heavy cream. When the mixture starts to simmer, thicken with cornstarch. Stir until mixture coats the back of a spoon. Remove from heat, let cook, then refrigerate while you make the cake.
Make the cake: Line a 9-inch springform pan with baking spray and parchment paper.
In a large bowl, whisk egg whites until stiff peaks form. Add egg yolks, one at a time.
Add sugar, flour, and cocoa powder. Fold in baking soda, then pour into springform pan.
Bake at 350F for 35 minutes.
Let cake cool, then cut a hole in the center. Scoop out the inside and set aside for later.
Mix strawberries into chilled filling, then spoon into the center of the cake. Cover the top of the filling with the scooped out cake.
Make the ganache: Pour hot heavy cream into chopped chocolate. Stir until smooth, then spread ganache over the cake. Refrigerate the cake for at least 1 hour.