Preheat the oven to 375 degrees. Line a large baking sheet with a Silpat baking mat or parchment paper. Set aside.
In a medium bowl, whisk together flour, baking powder, salt, and spices.
With a mixer, beat the butter and brown sugar until smooth and creamy, about 3 minutes. Add in the egg and vanilla. Mix until well combined. Add in the molasses. Mix until the molasses is combined with the other ingredients.
With the mixer on low, slowly add the flour mixture. Mix until just combined. Form the dough into tablespoon sized balls. Roll the dough in sugar. Place the cookie dough balls on the prepared baking sheet 2-inches apart. Bake for 8 minutes, don't overbake. Let the cookies sit on the baking sheet for two minutes, transfer to a cooling rack and cool completely.
While the cookies are cooling, make the white chocolate buttercream. Place white chocolate chips in a microwave-safe bowl and heat in 30-second intervals until chips begin to melt, about 1 minute. Stir and repeat, heating chips about 10 seconds at a time, until thoroughly melted. Stir until chocolate is smooth and no more lumps remain. Beat butter with an electric mixer on medium speed in a mixing bowl until fluffy. Beat in half of the powdered sugar, melted white chocolate, vanilla and milk until mixture is smooth and creamy. Slowly beat in remaining powdered sugar until smooth; if frosting is too stiff, add in a little more milk.
To make the sandwich cookies, spread or pipe white chocolate buttercream on the inside of one gingersnap. Top with another gingersnap. Continue making sandwich cookies. Store cookies in an air-tight container at room temperature.
Note-you may have extra buttercream, depending on how much you add between each cookie. You can keep the leftover buttercream in the fridge for up to a week.