Preheat your oven to 150C/300F. Grease a deep 10in round cake pan and line with parchment paper. Set aside.
Place the butter, white chocolate, sweetener, and sour cream into a heatproof bowl over a saucepan of simmering water. Heat, stirring occasionally, until the white chocolate has melted and all the ingredients are well combined.
Remove from the heat and add the vanilla and caramel extracts. Mix well.
Add the eggs and mix well.
Sift in the remaining dry ingredients and mix well.
Pour the cake batter into your prepared pan and smooth out the top.
Bake for 70-80 minutes, until the cake feels firm to the touch, and an inserted skewer comes out clean.
Set the cake aside on a rack to cool completely before frosting.
To make the ganache; add the white chocolate to a heatproof bowl and set aside.
Place the cream into a small saucepan and bring to a simmer, then pour over the chocolate. Leave to sit for 2 minutes then whisk together, ensuring that all the chocolate is melted.
Put the ganache into the fridge for 10-20 minutes to firm up slightly.
Using a serrated knife, trim the top of the cake to make it flat. Place it top side down onto a cake stand or platter.
Spoon the ganache over the cake, allow it to drizzle down the sides. For the piped effect, whip some of the ganache with a hand mixer for 2-3 minutes until it’s light and fluffy before piping over the cake.
Set the cake at room temperature for 15 minutes, for the ganache to set.