For the cookies, preheat oven to 350 degrees. Line a half size sheet pan with parchment paper.
Add the flour, baking powder, cream or tartar and salt to a bowl. Whisk and set aside.
Add cooled melted butter to a mixing bowl or stand mixer along with brown sugar, granulated sugar and vanilla. Mix for a minute.
Add the eggs one at a time. Mix, scrape the bowl down and mix again.
Slowly add the dry ingredients to the wet. Mix until fully combined.
Pour the batter into the parchment lined half size sheet pan. Use a spatula to spread it evenly. It will be thin. Cover in 2/3 of the cinnamon sugar.
Bake for 15-18 minutes until they're set in the middle.
Cool completely to room temperature.
For the ice cream add heavy whipping cream, sweetened condensed milk, vanilla bean paste and salt to a stand mixer fitted with a whisk attachment or using a handheld mixer. Beat together until medium peaks form.
Line a 9x13 dish with cling wrap with a lot of overhang.
Slice the cooled cookie in half vertically (right down the middle, top to bottom)
Add one half of the cookie to the 9x13 dish.
Top with all of the ice cream mixture. Spread out evenly.
Top with second half of the sheet pan cookie.
Tightly wrap in the overhanging cling wrap completely.
Freeze overnight.
Remove from the freezer, unwrap and place on a cutting board. To create clean lines cut the outside edge off.
Cut into 16 bars. Dredge the outside edges of each bar into the remaining cinnamon and sugar (optional).
Individually wrap in foil and store in the freezer.
Pro tip: slice each bar into 4 equal pieces. Somehow the thin slices taste even better!