For the Cookies:
230g (2 sticks) unsalted butter, at room temperature
300g Dark Brown Sugar
1 egg, at room temperature
1 tsp vanilla bean paste
300g (2 cups) all-purpose flour
½ tsp baking soda
1 tsp salt

Preheat the oven to 350˚f / 180˚c. Line three baking sheets with parchment paper.

To make the cookies, cream the butter and brown sugar in the bowl of a stand mixer fitted with the paddle attachment until light and fluffy, approximately 5 minutes. Add the egg and vanilla and mix well. Sift together the flour, baking soda and salt, and add to the mixer, mixing on low until combined. Scrape down the bowl to ensure the ingredients are evenly incorporated.
Using a 2 Tbsp Scoop, scoop out balls of cookie mixture - I had 8 per tray, and worked with one tray at a time, scooping the next while the first was baking. Place in the freezer for 5 minutes to firm up slightly. Remove the tray from the freezer. Roll the dough into balls, more is added during baking. Arrange evenly on the baking sheet, leaving room for spreading. If you are worried about your cookies holding hands in the oven, you can bake 6 per tray.
Bake the cookies for 12-13 minutes. After 10 minutes of baking, open the oven, and sprinkle the tops of the cookies generously with extra streusel. Remove from the oven, and allow to cool on the baking sheet for 15 mins before transferring to a cooling rack and allowing to cool completely.
Repeat the baking process with the remaining cookies.

Match up the cookies into pairs the same size. Remove the frozen yogurt from the freezer. Using a cookie cutter the same size as your cookies, dip the cutter in water and cut out discs of frozen yogurt. Sandwich between two cookies. Repeat until the remaining yogurt has been used up - you can mix and match pieces of yogurt to create a full circle to get a few extra, or you can use an ice cream scoop to scoop the leftover ice cream. Place on a parchment paper lined baking sheet, and transfer to the freezer to harden slightly. Store leftover cookie sandwiches in an airtight bag in the freezer.