Sift or mix flour, salt, and baking soda together in a bowl. Set aside.
Melt chocolate in a small bowl in the microwave for 20 seconds at a time. It will take a few times to get it all the way melted, but it's really important not to burn the chocolate. So keep it in short increments, stirring each time. When chocolate is fully melted, set aside to cool a bit.
In a large mixing bowl, cream together butter and sugar until light and fluffy.
Add yolks, one at a time, beating well after each addition.
Mix in cooled chocolate and vanilla.
Add flour mixture alternately with buttermilk in thirds (So add about a third of the flour mixture, followed by ⅓ cup of milk, and continue until both are entire mixed in). Beat after each addition until smooth.
Fold in stiff egg whites.
Bake at 350° in a lined and greased cake pan for 30-40 minutes, until cooked through. This recipe will make about three 8-inch layers or two 9-in layers.
For the Strawberry Buttercream
Add strawberries and lemon juice to a small saucepan. Over medium heat, bring mixture to a simmer, stirring often. Your strawberries will become soft enough that they will basically mash themselves while you're mixing. Simmer for about 20 minutes, or until you have about ⅓ cup of strawberry puree mixture.
If you don't want seeds or skins in your buttercream, strain the strawberry puree through a fine mesh strainer at this point. (I did not strain mine, but did run it through a food processor to make sure it was extra smooth) Allow to cool to room temperature
In a large mixing bowl, add butter and blackberry puree. Beat together with a mixer until mixture is smooth.
Add in the powdered sugar, and beat on medium-high until mixture is smooth and creamy.
Add your milk or cream and beat again to combine. Test your buttercream now. If it's still too thick, continue adding milk or cream, 1 tablespoon at a time until buttercream reaches desired consistency.
To assemble the cake
Place one of the cake layers on a serving plate or board. Frost with a third of the frosting. Add the next layer, and repeat.
Top with fresh strawberries and additional dark chocolate chunks.