Preheat oven to 200 degrees F. Line a large baking sheet with parchment paper and set aside.

Pop the popcorn with a popcorn maker or using the microwave (see notes). Spread the popped popcorn out onto the baking sheet. Sprinkle the sliced almonds on top of the popcorn.

In a medium saucepan over a medium-high heat, combine the butter, brown sugar, corn syrup, cream of tartar, and sea salt. Stir everything until combined with a rubber spatula and bring to a boil for 5 straight minutes, stirring every so often. Remove saucepan from the heat and quickly stir in the baking soda until completely combined. The caramel will puff-up - this is normal.

Pour the caramel over the popcorn and stir gently with a rubber spatula until most of the popcorn and almonds are coated.

Bake the popcorn for 1 hour, stirring at the 30 minute mark.

Once cooked, remove the popcorn from the oven and immediately sprinkle the top of it with large flaked sea salt (or regular salt). Stir the popcorn one last time and allow it to cool completely on the pan. Once cooled, break apart the large clusters with your hands.

Store the popcorn in an air-tight container (I used a large freezer bag). The popcorn stays fresh for a whole week!