To make the cornbread, mix the cornmeal, flour, caster sugar, baking powder and salt in a large bowl. Mix the buttermilk, melted butter and eggs in a jug. Make a well in the middle of the dry ingredients and gradually add the wet ingredients, stirring constantly to prevent lumps forming. Leave the mixture to stand for 20 minutes. Meanwhile, heat the oven to 190°C/gas mark 5.
Make the guacamole. Scoop the flesh of the avocado into a bowl and add the lime juice. Mash this with the back of a fork until fairly smooth then stir through the coriander, chilli and spring onions. Season to taste then cover with clingfilm and set aside.
Once the cornbread mixture has finished resting, pour the mixture into a 18 x 10 x 7cm non-stick loaf tin and bake for 40 minutes or until a skewer inserted in the middle comes out clean. Leave to stand for 10 minutes or until cool enough to handle, then place on a wire rack to cool. Slice 4 pieces and heat a griddle pan. Griddle the cornbread for around 1 minute on each side or until golden, then set aside and keep warm. (The remaining half of the cornbread will last for 2-3 days wrapped in clingfilm.)
Put a large pan of water on the heat and allow to boil. Add the white wine vinegar to the pan. Poach the eggs one at a time and transfer to a plate lined with kitchen paper to dry.
To serve, place a piece of cornbread on each plate and top with a quarter of the guacamole and a poached egg. Crumble a quarter of the feta on each of these then drizzle over sriracha to taste.