Pre-heat the Oven to 350˚F.
In the bowl of your electric mixer or with a hand held mixer whisk together 1 egg with 1/3 cup sugar until smooth.
Add in 5 1/2 Tbsp softened butter and mix on medium/high speed until creamy.
Add a generous pinch of baking soda along with the 1 1/3 cups of flour (1/3 cup at a time). Mix on Medium/high for 3-5 minutes or until the mixture is smooth.
If your cupcake tin is not non-stick then grease it with some butter. Use your hands to roll dough into gum-ball-sized pieces and place them into the mini-cupcake dishes. If the dough is too sticky add more flour. Smooth the dough balls evenly into the dishes making a large well in the dough (I didn't make mine deep enough and the dough rose too much).
Bake at 350˚F for 10-15 minutes, or until edges are golden. Tip cookies onto a large cutting board and turn them over. Let them cool to room temperature before filling. They will cool faster on a wire rack.