Preheat the oven to 350° F. Grease a 6-cup doughnut pan or 12-cup muffin pan with melted butterIn a large mixing bowl, whisk together the butter, milk, yogurt, eggs, vanilla, and sugar until combined. Add the flour, baking powder, baking soda, and salt, mixing until just combined. Stir in the blueberries. Dollop tablespoons size amounts of the jam around the batter, fold it into the batter, only stirring 2-3 times. You want to have steaks of jam. I used 1/3 cup jam, this will give you a more discolored doughnut.Spoon the batter into a zip-top bag and snip 3/4 inch off the corner off. Pipe the batter into each donut cup, filling 1/2-2/3 the way full. Bake 10 minutes. Remove and let cool 5 minutes, then run a knife around the edges to release, some berries need a little wiggle and can get stuck, so be gentle. Invert the doughnuts onto a cooling rack. Repeat with the remaining batter. If using a muffin pan, fill 2/3 the way full and bake 10-12 minutes.Mix the sugar and cinnamon in a shallow bowl. Working with one doughnut at a time, brush with the melted butter. Dip into the cinni-sugar mix, tossing to coat. Repeat with the remaining doughnuts. Enjoy!