First, combine the milk and lemon juice in a small bowl and set aside.
Preheat the oven to 175 C. Line an 8 inch square baking pan with non-stick baking paper, leaving a little overhang on two sides for easy removal.
Then in a mixing bowl, add the oil and peanut butter. Mix well with a silicone spatula or balloon whisk until combined.
Add the egg and vanilla, and mix till combined, followed by the water. Now pour in the soured milk, stir to combine and finally stir in the sugar until the mixture is fairly smooth.
Sift in the flour, baking soda, baking powder and salt. Fold to combine. The batter will look a little lumpy, but stir a little vigorously and it will become smooth. (If you're having trouble you can use a hand mixer here).
Pour the batter into the prepared pan. Bake for 25 to 30 minutes until the top is browned and a toothpick poked in the center comes out clean.
Let the cake cool in the pan for 15 minutes, then take out using the paper overhang and place on a wire rack to cool completely.
In the meantime, make the frosting. Heat the cream in a small saucepan until simmering, then pour over the chocolate (use a heatproof bowl). Let it stand for a couple of minutes, then stir until completely smooth. Allow to cool at room temperature for 10 mins, then transfer the bowl to the fridge, uncovered, for about 30 minutes until chilled through.
When ready to make the frosting, give the ganache a stir to loosen it up, then use a hand mixer to whip it for about 2 minutes, into a pale, creamy, fluffy frosting that forms soft rounded peaks when you lift the beaters (this is why whipping cream is recommended, regular cream won't produce the same peaks).
Spread immediately over the cooled cake and decorate with sprinkles if you like. Slice and serve! The cake, once frosted, should be stored in an airtight tin in the fridge and will last 4 to 5 days. Happy baking!