To make the brownie base:
Adjust oven rack to middle position; heat oven to 325 degrees. Cut parchment paper to fit the bottom and line the sides of a 9-inch springform pan. Spray lined pan with nonstick cooking spray.

Whisk to combine flour, salt, and baking powder in medium bowl; set aside.

Melt chocolate and butter in large heatproof bowl set over saucepan of almost-simmering water, stirring occasionally, until smooth.

When chocolate mixture is completely smooth, remove bowl from saucepan and gradually whisk in sugar. Add eggs on at a time, whisking after each addition until thoroughly combined. Whisk in vanilla. Add flour mixture in two additions, folding with rubber spatula until batter is completely smooth and homogeneous.

Transfer batter to prepared pan; using a small off-set spatula or bench scraper, spread batter evenly and smooth the surface. Bake until toothpick or wooden skewer inserted into center of brownies comes out with few moist crumbs attached, 25-30 minutes. Cool on wire rack to room temperature, about 1 hour (do not remove the springform ring or parchment collar).

To make the cake:
Adjust oven rack to middle position; heat oven to 325 degrees. Line the bottom of a 9-inch round cake pan with parchment and spray bottom and sides with baking spray.

Combine chocolate, cocoa powder, and hot water in medium heatproof bowl; set bowl over saucepan containing 1 inch of simmering water and stir with rubber spatula until chocolate is melted, about 2 minutes. Add 1/4 cup sugar to chocolate mixture and stir until thick and glossy, 1 to 2 minutes. Remove bowl from heat and set aside to cool.

Whisk flour, baking soda, and salt in medium bowl. Set aside. Combine buttermilk and vanilla in small bowl and set aside.

In bowl of stand mixer fitted with whisk attachment, whisk eggs and yolks on medium-low speed until combined, about 10 seconds. Add remaining sugar, increase speed to high, and whisk until fluffy and lightened in color, 2 to 3 minutes. Replace whisk with paddle attachment.

Add cooled chocolate mixture to egg/sugar mixture and mix on medium speed until thoroughly incorporated, 30 to 45 seconds, pausing to scrape down sides of bowl with rubber spatula as needed. Add softened butter one tablespoon at a time, mixing about 10 seconds after each addition. Add about one-third of flour mixture followed by half of buttermilk mixture, mixing until incorporated after each addition (about 15 seconds).

Repeat using half of remaining flour mixture and all of remaining buttermilk mixture (batter may appear separated). Scrape down sides of bowl and add remaining flour mixture; mix at medium-low speed until batter is thoroughly combined, about 15 seconds. Remove bowl from mixer and fold batter once or twice with rubber spatula to incorporate any remaining flour. Pour into prepared pan; smooth batter to edges of pan with an offset spatula.

Bake until toothpick inserted into center comes out with a few crumbs attached, about 30 minutes. Cool cake in pan 15 minutes, then invert onto wire rack and cool completely.

To make the Chocolate mousse filling:
Place stand mixer bowl and whisk attachment in the freezer or fridge.

Heat 1-inch of water in the bottom half of a double boiler over medium heat. Place the semisweet chocolate in the top half of the double boiler. Tightly cover the top with film wrap and allow to heat for 8 to 10 minutes. Remove from the heat and stir until smooth. Transfer the melted chocolate to a stainless steel bowl and set aside until needed.

Place heavy cream and sugar in the well-chilled bowl of an electric mixer fitted with a well-chilled whisk attachment. Whisk on high until stiff peaks form, about 1 1/2 minutes. Remove the bowl from the mixer.

By hand, whisk to combine 1/4 of the whipped cream into the melted chocolate until smooth and completely incorporated. Add the combined whipped cream and chocolate to the remaining whipped cream and use a rubber spatula to fold together.

To make the Ganache:
Heat the heavy cream and the butter in a 3-quart saucepan over medium high heat. Bring to a boil.

Place the semisweet chocolate in a 3-quart stainless steel bowl. Pour the boiling cream over the chocolate and allow to stand for 5 minutes. Stir until smooth. Reserve ~1/2 cup of ganache to use as glue for strawberries. Remaining ganache should be brought to room temperature (about 40 minutes).

To assemble the cake:
Spread mousse over top of the brownie base evenly. Use an offset spatula or bench scraper to smooth the top.

Place cake round over mousse, pressing down lightly. Chill for 1 hour.

Cover the edges of a round cake board with waxed paper strips. Remove springform ring and parchment collar. Use spatula to move cake to a cake board. Use a spatula to smooth the room temperature ganache over the cake top and sides, covering evenly. Place entire cake in fridge for ganache to set.

Once ganache has set, remove waxed paper strips from cake board. Heat reserved ganache in microwave for a few seconds. Use as glue to attach 15-16 chocolate covered strawberries. Serve chilled and store leftovers in the fridge. Cut the cake with a hot, dry knife, using the strawberries as a serving guide (1 strawberry per serving).