Pre-heat your oven to 180c and line a 20cm square baking tin with greaseproof paper.
Cream together the butter and sugar in a bowl until light and fluffy. Next add the eggs and vanilla extract and whisk together until the ingredients are combined.
Add the baking powder and flour and fold into the wet ingredients with a wooden spoon. Now add the natural yoghurt and chocolate chips and mix together.
Pour the batter into your baking tin and even off the top with the back of your wooden spoon. Bake for 40 minutes until your cake is a rich brown and a skewer pricked into the sponge comes out clean. Set your cake aside to cool whilst you make the vanilla frosting.
In a clean bowl whisk together the butter, icing sugar and vanilla. Whisk until a pale yellow, fluffy and smooth.
Once your cake has cooled, use a large knife (a bread knife works well) to neatly trim off the edges of your traybake. This creates a uniform square and a neat cross section of your chocolate chip sponge.
Use a spatula or palette knife to smooth the buttercream over the top of the chocolate chip cake, spreading right to the edges.
Serve in uniform squares