Add the currants and raspberries to a saucepan.

Heat over medium-high heat for about 10 minutes, until berries begin to break down and currants can be crushed with the back of a wooden spoon. Remove from heat.

Strain cooked fruit through a fine-mesh sieve, pressing fruit against the sides to extract the maximal amount of juice. This effort may take anywhere from 10-30 minutes, depending on the size and fineness of your strainer. Discard stems, seeds, and other solids.

Place cubed butter in a heatsafe bowl and set aside.

In a medium saucepan, add ¼ cup plus 1 tablespoon (75 ml) currant-raspberry juice, egg, egg yolks, and sugar, then whisk to combine.

Cook over medium heat for 6-8 minutes, stirring constantly to keep eggs from curdling, until mixture thickens enough to coat the back of a spoon, or reaches 160 F on a candy thermometer.

Remove from heat and strain through a clean, fine mesh sieve into the butter-containing bowl. Stir to combine. Cover with plastic wrap, pressing the plastic wrap down onto the surface of the curd to prevent formation of a skin. Refrigerate until set, at least 4 hours.

Curd may be stored in a glass jar up to 1 month in advance.

Preheat oven to 350 F (175 C). Grease and flour three 8-inch or 6-inch cake pans.*****

Whisk together flour, baking soda, baking powder, and salt. Set aside.

Beat together the sour cream, egg yolk, and vanilla in the bowl of a stand mixer or other large bowl.

Add the coffee, cassis, and butter to a medium saucepan. Place over medium heat until butter melts.

Whisk the cocoa powder and sugar into the cassis mixture. Once combined, remove from heat and let cool briefly.

Drizzle cassis mixture into the sour cream mixture while the mixer is running at low speed to combine. Scrape down sides of bowl.

Add the dry ingredients, beat on low or whisk until just combined.

Even divide batter amongst prepared pans (367 g batter each).

Bake for 18-19 minutes in 8-inch pans or 22-25 minutes in 6-inch pans, rotating once. An inserted toothpick should come out cleanly.

Let cool on wire rack for 10-15 minutes, then remove cake from pans onto parchment paper and continue cooling.

Place egg whites and sugar together in a bowl from a stand mixer, whisk together to combine. (if you don't have a stand mixer, hopefully you have a hand mixer and compatible metal bowl. This recipe will be nigh impossible to whisk completely by hand.)

Bring a few inches of water to a simmer in a medium saucepan over medium-high heat.

Place the mixer bowl over this saucepan to replicate a double boiler. The bottom of the mixer bowl must not touch the simmering water in the saucepan.

Over the course of about 15 minutes, whisk intermittently as the egg mixture heats to 160 F (70 C) on a candy thermometer. If you don't have a thermometer, it should be hot when tested on the inside of your wrist (don't burn yourself).

Once hot, carefully fit the mixer bowl onto your stand mixer. Using the whisk attachment, beat the egg white-sugar liquid on high speed for 8-10 minutes, until medium-stiff peaks are formed. Once complete, the mixer bowl should have cooled to room temperature and there should be no residual heat escaping the meringue.

Swap the whisk for the paddle attachment. Turn the mixer to low speed and begin adding the room temperature butter, 2-3 tablespoons at a time. For a large recipe, this will take about 30 minutes. Once fully incorporated, add the vanilla, and beat at low speed to incorporate. Turn the mixer up to medium-high and beat until silky smooth, about 3-5 minutes.

Store leftover buttercream in the refrigerator for 10 days or in the freezer for 2 months. Thaw frozen buttercream in the refrigerator and bring stored buttercream to room temperature before using.

Place chopped chocolate in a heat-safe bowl.

Bring the cream to a simmer slowly over medium-low heat in a small saucepan. Take care not to scorch the cream.

As soon as the cream begins to simmer, pour it over the chopped chocolate.

Let cream and chocolate stand 30 seconds, then whisk together until smooth (this should take a few minutes).

Let cool until it has thickened some but is still runny with a spreadable consistency.

If not using immediately, store in the refrigerator for up to a week. Bring to spreadable consistency by microwaving in short 20 second intervals, stirring in between, or reheat in a double boiler.