Preheat oven to 180C/350F. Line the bottom and sides of a glass jar (10" circumference x 4" tall, not skewed at the neck) with parchment parchment paper.

In a bowl, combine the dry ingredients: flour, sugar, cocoa, baking soda and salt. Stir to make sure it's evenly distributed.

In the center, add the liquids: water, oil and vanilla. Stir to make a smooth batter.

Transfer to the prepared glass jar, move jar onto a small baking sheet. Bake at 180C/350F for 30-35 minutes, or until a skewer inserted into the center comes out clean.

Transfer to a wire rack to cool slightly, and while still warm to the touch, wiggle the cake out from the jar using the parchment paper. Let cool completely on the wire rack.

In the meantime, make the frosting. Heat the coconut milk until warm and pour over the chopped chocolate. Let sit until the chocolate melts in the heat. Stir to make a smooth chocolate ganache.

Add the icing sugar, one spoon at a time, and with a small whisk or a spoon, beat until smooth. Refrigerate for 2-3 minutes until slightly firm (piping consistency).

Transfer the icing to a piping bag fitted with a leaf tip (if piping the rose). Else use a plain round tip.

Divide the cake into three pieces. Spread jam on one side of the bottom two slices.

Pipe over some frosting between the two layers, spread with a small palette knife. Sandwich the layers together.

Pipe more frosting on the sides and the top. Smooth down best you can. You can refrigerate the cake to make it easier to work with, if needed.

With the remaining frosting, pipe a rose. (If you are not an expert at piping roses, do this step before frosting the cake. Save one best rose, and scoop the remaining roses back into the piping bag).

Transfer the rose to the top of the cake. Alternatively, you can freeze the rose to make it more solid, and easier to transfer.

Refrigerate the cake (for an hour) for the flavors to mingle. Bring back to room temperature and serve.