Preheat the oven to 350 °F (177 °C).

Prepare the pan by buttering or spraying with oil the insides of 9" cake pans and lining the bottom with a circle of parchment paper.

Chop the chocolate into mini-chip sizes and add to a large mixing bowl with black cocoa and boiling water. Whisk until melted.

Add brown sugar and sour cream, letting it sit for about 5 minutes.

Add canola oil, salt, and eggs, whisking until combined. Then add flour and baking powder, whisking until no traces of flour remain.

Evenly distribute the batter into the prepared pans.

Bake for about 50-55 minutes until the centers do not jiggle and a skewer inserted has a few moist crumbs.

Cool the cake in the pan on a wire rack. Once cool enough, remove from pans and peel off parchment.

For the buttercream, make the meringue syrup by mixing egg whites and sugar, then microwaving until sugar is dissolved.

Cool the meringue syrup and then whisk softened butter until lighter in color. Add powdered sugar, vanilla extract, and salt, mixing until smooth.

Combine the meringue syrup with the butter mixture in increments, mixing until fully incorporated.

For the chocolate mousse, chill mixing bowl and whisk, then melt chocolate with butter and salt.

Whip heavy cream and sour cream until thick, then add powdered sugar and mix until soft peaks form.

Fold in the melted chocolate until fully integrated.

Chill the mousse before using it to fill cakes.

Once cakes are cooled, stack and frost as desired.