Preheat over to 350°F. Melt butter in a small saucepan and set aside.

In a bowl mix sugar, flour, cocoa, baking powder, and salt.

Make a well in the middle of the dry ingredients and crack two eggs into it. Add vanilla.

Using Danish dough whisk, fork, or wooden spoon, whisk eggs well then stir into dry ingredients while streaming in melted butter. Mix the batter together until well combined. Add in your add-ins at this point, mixing together.

Spread into a greased 8x8 pan.

Bake for about 35-40 min, or until a butter knife inserted in the center comes out clean. Don't over bake.

Cool and then chill until ready for mousse and toppings.

Chop chocolate into small pieces and set aside.

To a mixing bowl, add egg yolks, sugar, water, and salt and whisk well.

Pour into sauce pan and continue to whisk on low-medium heat until thick and hot to the touch.

When custard is thick and hot, remove from heat and add chocolate and butter, and whisk until chocolate is melted.

Whisk occasionally to further cool the mixture to just above room temperature. Caution, if cooled too long, the mixture may get too firm to fold into whipped cream. Now is not the time to walk away!

Whisk or blend chilled cream until peaks start to form.

Transfer around a third of chocolate custard into whipped cream folding with spatula until almost incorporated then add remaining chocolate mix continuing to fold until gone. Keep it fluffy!

Spread mousse evenly over chilled brownies.

Chill pan for at least 20 minutes.

Cut cake into either 8 pieces if you intend to double stack, or 4 larger pieces for a flatter slice.

Carefully stack two slices on a plate, and top with whipped cream and berries or any other desired toppings.