
Preheat the oven to 350°F. Line a 13" x 18" rimmed baking sheet (half-sheet pan) with parchment and lightly grease the parchment.
To make the bars: In a large microwave-safe bowl, melt the butter and whisk in the brown sugar. Add the salt and vanilla, stirring until smooth.
If the mixture is uncomfortably hot to the touch, allow it to cool a bit before adding the eggs all at once. Whisk until smooth.
Stir in the flour, oats, baking powder, and spices. Substitute ingredients Blog What can I substitute for brown sugar? By PJ Hamel
Use a small offset or flexible spatula to spread the batter evenly in the prepared pan, making a thin layer that reaches the corners.
Bake the cookie bar for 12 to 14 minutes, until it’s set in the middle and just starting to turn golden brown; rotate the pan halfway through to ensure even baking. Remove the cookie bar from the oven and cool completely before assembling the sandwiches.
To make the ice cream: In a stand mixer fitted with the whisk attachment, beat the heavy cream, condensed milk, freeze-dried strawberry powder, vanilla, and salt on medium speed until medium peaks form.
Fold the diced strawberries into the whipped mixture.
To assemble: Remove the cookie bar from the baking sheet and cut it in half crosswise to make two pieces approximately 8 1/2" x 12" each.
Line a 9" x 13" pan with plastic wrap, allowing it to generously overhang on all sides. Place one of the bars in the pan, bottom-side up.
Top the bar with the whipped strawberry mixture. Spread it toward the corners, leaving a 1/2" border on all edges bare.
Place the other cookie bar on top of the ice cream bottom-side down, gently pressing down to evenly spread the ice cream to the edges. It’s OK if some of the ice cream oozes out the sides.
Using the overhanging wrap, completely cover and tightly wrap the assembled sandwich. Place the pan in the freezer overnight or for at least 12 hours.
To serve: Remove the sandwich from the freezer. Unwrap and place it on a cutting board. For the neatest look, trim the edges of the sandwich (baker's treat!). Cut into bars that are the size and shape of your choice and serve immediately.
Storage information: Store the cut homemade ice cream sandwich bars, individually wrapped, in the freezer for up to 3 months.