Line a cookie sheet with parchment paper. Arrange the saltines in a square on the lined cookie sheet and set aside.
Put the butter, sugar, salt and 1/4 cup water in a large microwave-safe bowl but do not stir.
Microwave on high until the mixture just begins to turn lightly golden, about 8 minutes. Continue to microwave in additional 30-second or 1-minute increments if needed to achieve the golden color.
Carefully pour the cooked candy over the saltines on the cookie sheet. Let stand 1 minute. Sprinkle the chocolate chips over the hot toffee and let stand until the chips are shiny, 1 to 2 minutes. Use an offset spatula to spread the melted chips over the toffee. Sprinkle with sea salt, if using. Chill in the fridge until the chocolate is firm, about 20 minutes. Break the candy into bite-sized pieces.