【Chocolate Sponge Cake】
☆Preparation
Line an 18cm square cake pan with parchment paper.
Melt chocolate and butter in a microwave (600W for 40-50 seconds), add warm milk (about body temperature, warmed 600W for 10 seconds), and mix until smooth. Keep warm at 50℃ with hot water.
Preheat the oven to 190℃.
Mix cake flour, cocoa powder, and baking powder.
Separate the egg yolks and egg whites.

Beat the egg whites with a hand mixer on high speed until they turn white, then add granulated sugar in 3 parts and beat until stiff peaks form.
Add the egg yolks and mix on low speed for 3 minutes. Scrape down the edges and mix slowly on low speed for another 1 minute.
Sift in the cake flour, cocoa powder, and baking powder, and gently fold it in with a whisk until approximately combined.

Add a small amount of batter to the chocolate, and mix well. Return it to the cake batter and fold it in about 20 times. Scrape down the edges and bottom from time to time.

Pour the batter into the cake pan, and gently mix the surface to remove any unevenness.
Lower the oven temperature to 170℃ and bake for 27 minutes. Remove from the oven, and immediately drop the pan lightly a few times to prevent shrinkage.
Remove the cake from the pan and peel off the paper. Cover it lightly with a wrung wet cloth (or kitchen paper) and let it cool well.
After cooling, remove the cloth and wrap it in plastic wrap.

【Preparing the Mousse ring】
Attach mousse film to a 15cm square mousse ring. The height of the mousse film should be 6-7cm.

【Syrup】
Add hot water to granulated sugar and mix until dissolved. Heat in the microwave for 20 seconds (600W) and bring to a boil. Add rum and mix, then let it cool.

【Ganache】
Cut the chocolate into small pieces and put them in a heat-resistant bowl. Heat the heavy cream in the microwave until just before boiling (600W 40 seconds), then add it to the chocolate. Wait about 30 seconds and then stir to melt. When it's mostly melted, add the butter and heat it a little more in the microwave (600W for 20 seconds), and stir well until smooth.

【Assembling the base】
Slice the sponge into 1cm thickness. Use two slices and cut them both into 15cm squares.
Place the first sponge slice in the mousse ring and brush it with syrup. Pour ganache (at room temperature) on top. Place the second sponge slice on top and gently press it down. Brush with syrup. Cover with plastic wrap and refrigerate.

【Jam jelly】
Sprinkle gelatin over water and let it soak. To prevent lumps, add water first and then the gelatin.
Add hot water to the jam and mix well. If the jam is already runny, you don't need to add boiling water.
Melt the gelatin in the microwave (about 10 seconds at 600W), being careful not to let it boil. Add it to the jam and mix well.

Chill the jam mixture with ice water, stirring occasionally, until it thickens. Be sure to chill it until it's firm, or it will flow out from the edge of the sponge.
Pour the mixture over the sponge and chill in the refrigerator until set. (No need to cover with wrap)

【Chocolate Mousse】
Make sure the jam jelly is set before making it.
Melt the chocolate in a double boiler (water temperature 50-60℃) and let it cool.
Whip the heavy cream while cooling ice water until stiff peaks form, then put it in the refrigerator.

Make the egg yolk cream. Beat the egg yolks with a hand mixer on high speed for about 3 minutes until they turn whitish.
In a small pot, heat the water and granulated sugar until it reaches 115-120℃. Occasionally, use a wet brush to remove the splattered syrup on the inside of the pot.

Beat the egg yolk while drizzling the syrup in a thin stream. Beat until cool to room temperature, around 5 minutes.

Add 1/3 of the whipped cream to the chocolate and roughly mix with a whisk. Add the egg yolk cream, mix roughly, then add the rum and mix until smooth. Add the remaining whipped cream in two batches and fold it in with a spatula.

【Decoration】
Remove the ring and peel off the film.
Sprinkle cocoa powder over the top. Use a warmed knife to cut the 4 sides and smooth the sides.
Top with strawberries and blueberries, and lightly sprinkle with powdered sugar. Decorate with chocolate as desired, and it's done.