Preheat oven to 425°. In a small mixing bowl melt the butter. Allow to cool for a few minutes.
Line your muffin pan (12 cups) with cupcake wrappers.
In a large mixing bowl, combine the flour, sugar, lemon zest, baking powder, baking soda and salt. Whisk to combine.
In a large measuring cup, combine the sugar, cooled butter, yogurt, buttermilk, eggs, lemon juice and vanilla extract. Whisk until well combined.
Add the wet ingredients to the dry ingredients. Mix until just combined. Be sure not to overmix.
Gently fold in the blueberries and mini chocolate chips.
Use a large cookie scoop to add batter to the muffin pan. Fill each muffin cup halfway.
Place 1 teaspoon of blueberry jam onto each muffin.
Add more batter to each muffin. The cups should be filled to the top of the cupcake wrapper. Sprinkle the batter with coarse sugar.
Bake muffins at 425° for 5 minutes. Lower the temperature to 375° and bake for an additional 16-18 minutes or until the edges are golden brown and the middles are baked through.
Remove muffins from the oven and allow the muffins to cool.