Prep Oven & Tin: Preheat oven to 180°C (350°F). Grease and line a standard baking sheet with overhanging baking paper; grease the paper too.
Make 'Buttermilk': Stir apple cider vinegar into soy milk; set aside.
Cream Fat & Sugar: Whisk vegan butter/margarine and caster sugar until light and fluffy (stand mixer, hand whisk, or by hand).
Add Wet Ingredients: Whisk in the 'buttermilk' mixture and vanilla extract until just combined (mixture may look slightly split).
Add Dry Ingredients: Add flour, baking powder, and salt. Mix on low speed or fold gently until *just* combined – do not overmix.
Bake Sponge: Spread batter thinly onto the prepared baking sheet. Bake for 12-15 minutes, or until a toothpick comes out clean.
Cool & Cut Cake: Cool in tin for 10 mins, then turn out onto a wire rack to cool completely. Cut the cooled cake in half crosswise.
Make Coffee Syrup: Dissolve brown sugar in hot, strong coffee. Let cool completely. Stir in rum, if using.
Make 'Mascarpone': Whisk vegan cream cheese, melted refined coconut oil, and maple syrup until smooth. Add sifted icing sugar and whisk on high speed for 5-8 minutes until very light and airy, scraping down sides as needed.
Assemble Layer 1: Place one cake half in your serving dish (trim/squish to fit). Brush evenly with half the coffee syrup. Spread half the 'mascarpone' cream over the top.
Assemble Layer 2: Place the second cake half on top. Brush with the remaining coffee syrup. Spread the remaining 'mascarpone' cream evenly over the top.
Chill: Cover the dish tightly and refrigerate for at least 2-3 hours, or preferably overnight, to set firmly.
Serve: Dust generously with cocoa powder just before serving.