Preheat oven to 350°F (175°C) and prepare a baking sheet with parchment paper.
Cream softened butter and granulated sugar together until light and fluffy using an electric mixer.
Add egg and vanilla extract to the creamed mixture and beat until combined.
In a separate bowl, whisk together flour, baking soda, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture and stir in the milk until a soft dough forms.
Divide the dough into portions and tint each with gel food coloring for pastel shades.
Roll tablespoon-sized dough balls and place them on the prepared baking sheet.
Bake cookies for 8 to 10 minutes until edges are set but centers remain soft.
Cool cookies on the baking sheet for 2 minutes, then transfer to a wire rack to cool completely.
Prepare the filling by beating softened butter until smooth; add powdered sugar, vanilla, and a pinch of salt.
Mix in milk or cream until the filling is spreadable and fluffy.
Spread or pipe the filling onto the flat side of one cookie and top with another.
Let the cookie sandwiches rest for 10 minutes to allow the filling to set before serving.