Preheat your oven to 350°F (175°C) and grease or line a muffin tin with paper liners.

In a large mixing bowl, whisk together the sifted flour, 3/4 cup of sugar, baking powder, baking soda, and salt until well combined.

In another bowl, combine the melted butter, buttermilk, eggs, and vanilla extract. Whisk until smooth.

Pour the wet ingredients into the dry ingredients, and gently fold them together using a rubber spatula until just combined. Be careful not to overmix; a few lumps are okay.

Using an ice cream scoop, fill each muffin cup about halfway with the batter.

Add about 1 tablespoon of jelly into the center of each muffin cup, then cover with additional batter until the cups are about 3/4 full.

Bake in the preheated oven for 18-20 minutes or until a toothpick inserted into the center comes out clean.

Once baked, remove the muffins from the oven and let them cool in the tin for 5 minutes before transferring them to a cooling rack.

While the muffins are cooling, mix together the remaining 1/4 cup of sugar and ground cinnamon in a shallow bowl.

Once the muffins are cool enough to handle, roll each muffin in the cinnamon sugar mixture until fully coated.

Enjoy your delicious jelly doughnut muffins warm or at room temperature!