If you have 3 x 8-inch tins (and your oven is big enough to fit all three tins on one shelf), you can make the whole amount of batter at once and bake the 3 layers simultaneously. Alternatively, you can bake the layers one at a time (using the single layer quantities listed in italics in the ingredients list), or bake two layers at once (using the 2 layer quantities listed in bold), then make the final layer individually (which is what worked best for me).
Preheat the oven to 150°C (conventional oven, not fan-assisted). Grease the baking tin(s) with Stork and line the base(s) with a circle of greaseproof paper. Set aside.
Place the brewed espresso, boiling water and instant coffee granules in a large measuring jug. Stir to dissolve.
Add the milk, sour cream and vanilla extract and whisk until smooth. Allow to cool slightly, then add the eggs and whisk again until fully incorporated. Set aside.
Sift together flour, bicarbonate of soda and salt. Set aside.
Using a stand mixer fitted with the paddle attachment, cream Stork, caster sugar and light brown muscovado sugar together on high speed until well-combined and fluffy.
With mixer on medium-low, gradually add 1/3 of the dry ingredients. Continue mixing until just incorporated.
With mixer still on medium-low, gradually add 1/2 of wet ingredients. Continue mixing until just incorporated.
Repeat this process with the next 1/3 of the dry ingredients, followed by the remaining 1/2 of the wet ingredients and finishing with the final 1/3 of the dry ingredients.
Turn the mixer speed up to medium and mix briefly to ensure everything is incorporated and batter is smooth.
Divide the batter between the prepared tin(s) and bake in preheated oven for 60-65 minutes, until a skewer inserted into the centre comes out clean.
Allow the cakes to cool completely in the tins, then carefully run a palette knife around the edges to loosen them. Use a large, serrated palette knife to level the tops of the cakes while still in the tins, then carefully turn out onto wire racks.