Preheat the oven to 350F / 175C / 155fan. Generously grease a bundt tin with butter. If using a rectangular loaf pan or springform, line the bottom and sides with baking / parchment paper.
Melt the butter in a saucepan or microwave until just warm. Allow to cool for a couple of minutes and add the egg yolks. Whisk with a balloon whisk or with an electric mixer to combine. Then whisk through the yoghurt and lemon juice.
Place all the dry ingredients in a clean mixing bowl and stir to combine. Also add in the lemon zest, reserving 1 tsp for decorating the cake later.
Add the wet ingredients to the dry and fold together with a spatula or blend with an electric mixer.
Whisk the egg whites in a clean mixing bowl using an electric mixer until stiff peaks form.
Fold the egg whites through the batter until smooth using a spatula (no electric blenders here!). Don’t over-mix or you will deflate the eggs.
Spoon the batter into the bundt tin and level with a spatula.
Bake in the oven for about 45 - 50 minutes until golden and you can insert and remove a skewer without crumbs sticking. Mine took 48 minutes. Run a sharp knife around the edge of the tin. DO NOT REMOVE FROM THE TIN UNTIL FULLY COOLED.
Melt the coconut butter and oil using the bain marie method - in a glass proof bowl over a pan of simmering water. Remove from the heat and stir through the vanilla extract. Remove the lemon cake from the mould and drizzle over the coconut butter. Option to top with lemon zest and slices for decoration.