Special equipment: A pastry bag fitted with a large star tip
Make the cookies:
Preheat the oven to 350 degrees F. In a medium bowl, whisk together the oats, flour, cinnamon, baking powder, salt and baking soda.In a large bowl, beat the butter and sugars together with an electric mixer on medium until light and fluffy, 3 minutes. Add the egg and vanilla and beat to combine. Beat the flour mixture into the butter mixture. Fold in the raisins and walnuts. Line 2 baking sheets with parchment paper. Scoop the dough in 2-tablespoon scoops and place on the lined baking sheets at least 2 inches apart. Bake until the cookies are set and golden brown, 12 to 14 minutes, rotating the sheets halfway through. Transfer the cookies on the sheets to racks to cool completely. Meanwhile, prepare the cream: In a large bowl, beat the cream cheese and butter together with an electric mixer on medium until smooth and creamy, about 2 minutes. Beat in the sugar on low, then beat in the vanilla extract and salt. Transfer the cream to a pastry bag fitted with a large star tip. Transfer half of the cookies to a work surface, flat-side up. Top each cookie with some of the frosting. Top with another cookie. Chill the cookies for 20 minutes to firm up the frosting before serving.