Combine softened butter, egg, both sugars, vanilla and beat well until creamy.
Add in the oats, flour, cinnamon, baking soda, salt, and beat until well combined. Then, stir in the chocolate chips.
Using a tablespoon scoop, measure out about 18-20 mini balls and flatten slightly. Place cookie dough balls into a bowl or on a tray, cover and place in the fridge for at least 2 hours or up to 5 days. These must chill, see note in post above.
Once the cookies have chilled, preheat the oven to 350F and line a baking sheet with parchment paper. Place cookie dough balls on the sheet and separate 2 inched apart. Bake for about 9 minutes or until edges and tops have set. Should be slightly pale. Let cool completely.
Beat butter on high until creamy. If using a stand or handheld mixture, beat on high for about 1 minute. Add in powdered sugar and beat on medium for 1 minute. Pour in the heavy cream and the vanilla. Beat on high for 2 minutes or until fluffy. Add salt if desired.
If filling is too thick, add a teaspoon or 2 of heavy cream.
Speed about 2 teaspoons of cream filling on the bottom side of hand of the cookies. Top with remaining cookies, right side up.
Store cookies at room temperature for up to 2 days in an airtight container. Store in fridge after 2 days to keep cream filling very fresh.