Combine cocoa powder and hot water to make a paste. In a bowl, sift together flour, baking powder, baking soda, and salt. Cream the softened butter and sugar until fluffy. Add eggs one at a time, then mix in the cocoa paste, vanilla, and oil. Alternately add the flour mixture and buttermilk, mixing until just combined.

Divide the batter evenly between greased and floured cake pans. Bake at 350°F (175°C) for 25–30 minutes or until a toothpick inserted comes out clean. Let cool completely. Once cooled, stack the layers and use a serrated knife to gently carve the cake into a rounded, dome shape.

Beat chilled heavy cream with powdered sugar and vanilla until stiff peaks form. Slice each cake layer horizontally if needed for more layers. Spread whipped cream between layers, adding some fresh fruit if desired. Cover the entire dome with the remaining whipped cream.

Arrange sliced strawberries, blueberries, raspberries, and mandarin oranges over the top and sides of the dome. Refrigerate for at least 1 hour before serving for the best texture and flavour.