Whip heavy cream in a mixing bowl until stiff peaks form (3-4 minutes). In a separate bowl, combine sweetened condensed milk, vanilla extract, and a pinch of salt. Gently fold whipped cream into the mixture until just combined. Pour into a lined baking pan and freeze for at least 3 hours until firm.

In a medium bowl, whisk together flour, cocoa powder, baking powder, and salt. In a large bowl, beat butter and sugars until creamy (2-3 minutes). Add egg and vanilla; mix until blended. Gradually add dry ingredients.

Chill the cookie dough on a floured surface, rolling to 1/8-inch thickness. Cut into rectangles and chill in the refrigerator for 25-30 minutes.

Preheat oven to 350°F (175°C). Place chilled rectangles on a baking sheet and bake for 8-10 minutes until edges are firm and centers are soft. Cool completely.

Cut frozen ice cream filling into rectangles. Assemble sandwiches by placing a rectangle of ice cream between two cookies. Wrap in plastic wrap and freeze for at least 8 hours.

Once set, unwrap and serve cold. Enjoy your homemade ice cream sandwiches!