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Preheat the oven to 350°F with a rack in the center. 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 blondie bars: In a large bowl, combine the mix and baking soda. Add the butter and eggs, stirring until smooth.
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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 and ensures the butterscotch chips are evenly distributed.
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Bake the blondie bar for 14 to 18 minutes, rotating halfway through, until it’s set in the middle and the top is lightly golden brown. Remove the blondie bar from the oven and cool completely in the baking sheet before assembling the sandwiches.
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To make the filling: In a stand mixer fitted with the whisk attachment, beat ingredients on medium speed until medium peaks form.
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To assemble: Remove the blondie bar from the baking sheet and cut it in half crosswise to make two pieces each approximately 8 1/2" x 12".
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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 bar halves in the pan, bottom-side up.
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Top the bar with the whipped cream mixture. Spread it toward the corners, leaving a 1/2" border on all edges bare.
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Place the other blondie bar on top of the filling bottom-side down, gently pressing down to evenly spread the whipped cream mixture to the edges. It’s OK if some of the filling 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 (we recommend 2" x 3") and serve immediately.
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Storage information: Store the cut blondie ice cream sandwiches bars, individually wrapped, in the freezer for up to 3 months.
