Preheat oven to 180°C. Grease a 20cm springform cake pan and line the base and sides with baking paper.
Place butter and instant coffee in a small saucepan over low heat, stirring until butter has melted and coffee has dissolved. Transfer to a medium bowl with the sugar, vanilla and eggs and whisk until well combined. Add flour and cocoa and mix until just combined. Pour into the prepared pan, smooth the top, and bake for 28-30 minutes or until the edges are set but the centre still feels soft (a skewer inserted in the middle should come out wet). Cool completely in the tin.
To make the coffee syrup, place the coffee and sugar with 1 cup (250ml) water in a small saucepan over medium heat, stirring to dissolve the sugar. Simmer for 7-8 minutes until mixture has reduced slightly and is syrupy.
Remove cake from pan and drizzle with coffee syrup. Serve with double cream.