Place milk in a large saucepan over medium low heat. While the milk is warming, place egg yolks and sugar in bowl of a stand mixer and beat on medium-high with paddle attachment until mixture turns from yellow to very pale yellow. This will take about 2-3 minutes. In the meantime, keep an eye on the milk. Turn off heat once milk begins to steam and before it boils.
Add cornstarch, cocoa, and salt to pale yellow egg yolks. Mix on low until combined. While the mixer is on low, very slowly add all of the hot milk. Place mixture back in saucepan and place over medium-low heat to thicken it. Make sure to constantly stir the mixture to ensure the mixture doesn't burn on the bottom.
Once thick like pudding, removed from heat. Add butter, chopped chocolate, and vanilla extract. Whisk together until melted together. Finally, add heavy cream and whisk together. Let cool to room temp and then place in a bowl. Cover with plastic wrap, making sure the plastic touches the pudding. Chill until cold, about 4 hours.
Once the pudding is cold, prepare the whipped cream. Whip the heavy cream with ¼ cup granulated sugar until stiff peaks form. Layer pudding, whipped cream, and chopped strawberries in parfait glasses. Use piping bags to help layer them neatly. You can do one layer of each or two layers. Serve immediately or place in refrigerator and serve within a couple hours.