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Preheat the oven to 350°F. Line a 13" x 18" rimmed baking sheet (half-sheet pan) with parchment and lightly grease the parchment.
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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.
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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.
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Stir in the flour, oats, baking powder, and spices.
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Use a small offset or flexible spatula to spread the batter evenly in the prepared pan, making a thin layer that reaches the corners.
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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.
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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.
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Fold the diced strawberries into the whipped mixture.
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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.
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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.
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Top the bar with the whipped strawberry mixture. Spread it toward the corners, leaving a 1/2" border on all edges bare.
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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.
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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.
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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.
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Storage information: Store the cut homemade ice cream sandwich bars, individually wrapped, in the freezer for up to 3 months.
