Do ahead: make the lemon curd.

• Preheat the oven to 350 F. Butter and flour a loaf pan and line with parchment paper.
• In the bowl of a stand mixer fitted with a paddle attachment, add the flour, butter, sugar, zest, salt and baking powder. Turn the mixer on low (cover with a tea towel to prevent the flour from making a mess) and run the mixer until the mix is sandy, about 5 minutes. The goal is for the butter to completely cover the flour.
• Add one egg and turn the mixer on medium speed, allowing the egg to fully mix in before adding the next. Add the kefir and beat until airy, fluffy and light, about 3-5 minutes. Fold in the blueberries.
• Spread a quarter of the batter into the prepared pan in an even layer. Dollop in some of the lemon curd and smooth into an even layer. Spread another layer of batter and repeat. Don’t leave any curd for the final layer, let it be pure batter. I had three layers of lemon curd in mine and four layers of batter.
• Sprinkle some sugar onto the top of the cake (a couple of tablespoons - optional) and bake until a cake tester comes out clean, about 60 minutes. Because this loaf cake is heavy on the fillings it will sink when it comes out of the oven - this is normal. Enjoy warm or wait until it’s at room temperature and place in an airtight container to keep it from drying out and enjoy later.