Preheat the oven to 180°C (350°F), Gas Mark 4. Lightly grease the baking tin and line the base with a piece of parchment paper.
For the cake, place the flour, baking powder and salt into a large bowl and whisk briefly to combine. Add the butter and sugar to a separate large bowl and, using an electric mixer, beat together on medium-high speed for about five minutes until light and fluffy. Add the eggs, one at a time, beating until fully combined before adding another. Add the vanilla and mix briefly to combine. Scrape the batter into the prepared tin and level out. Add the flour mixture in three additions, alternating with the sour cream, starting and finishing with the flour.
Bake for 35–40 minutes or until the cake springs back to a light touch. Leave to cool in the tin.
For the fudge frosting, place the butter in a large bowl and use an electric mixer to beat on high speed for a couple of minutes or until creamy and smooth. Add the sugar, golden syrup (or honey) and cocoa powder and beat on high speed for five minutes or until light and fluffy. Add the hot water, sour cream and vanilla to a small jug and whisk together. Add the sour cream mixture to the bowl and mix on medium speed until combined. It will look separated for a while but will come back together as a smooth frosting. Add the melted chocolate and beat briefly until smooth and silky.
Spread the frosting over the cake, finishing with a generous amount of sprinkles, which as far as I am concerned are mandatory. Cut into squares to serve. Store in a sealed container for up to three days.