Preheat your oven to 350°F (175°C) to ensure even baking and good rise.
Grease your mini muffin tin with butter or line it with mini muffin liners to prevent sticking.
In a large mixing bowl, whisk together the flour, baking soda, salt, and 1/2 cup granulated sugar to evenly distribute the leavening agent.
In another bowl, cream together the room temperature butter and remaining 1 cup sugar until light and fluffy, incorporating air for a lighter muffin.
Stir in the orange zest and juice from the orange into the butter-sugar mixture to create a fragrant base.
Add the sour cream to the orange mixture and combine well to add moisture and richness.
Gradually fold the dry ingredients into the wet mixture, being careful not to overmix; a few lumps are okay.
Gently fold in the optional raisins and walnuts if using, adding texture and flavor.
Spoon the batter into the prepared muffin tin, filling each cup about three-quarters full to allow room for rising.
Bake in the preheated oven for 12-15 minutes, or until a toothpick inserted into the center comes out clean.
Remove muffins from the oven and let cool in the tin for about 5 minutes before transferring to a wire rack to cool completely.