Preheat oven to 350°F (175°C). Grease and line an 8-inch round or springform pan.
In a bowl, whisk together flour, cocoa powder, baking powder, and salt.
In another bowl, beat eggs and sugar until pale. Add oil, milk, and vanilla, stirring to combine.
Gently fold the dry ingredients into the wet mixture until smooth.
Pour the batter into the prepared pan and bake 18–20 minutes, or until a toothpick comes out clean. Allow the cake to cool completely in the pan.
Melt the dark chocolate chips in a heat-safe bowl (microwave in short bursts or over a double boiler). Let cool slightly.
In a separate bowl, whip the cold heavy cream and vanilla until it holds stiff peaks.
Gently fold the whipped cream into the melted chocolate until smooth and airy. Spread this layer evenly over the cooled cake base. Chill 30 minutes to set.
Melt the white chocolate chips and let them cool slightly. Whip the cold heavy cream and vanilla until stiff peaks form.
Fold the whipped cream into the melted white chocolate gently. Spread this over the dark chocolate layer. Chill another 30 minutes or until set.
Whip the remaining heavy cream with powdered sugar until stiff peaks form. Pipe or spread this on top of the white chocolate layer, then garnish with chocolate curls if using.
Refrigerate the assembled cake at least 2–4 hours before slicing for best texture.
