Preheat oven to 350 degrees. Prepare 1/2 sheet pan (interior measures about 16.5x11.38 x1 inches) by spraying with nonstick spray. Warning - if you don't have a pan this size, you're better going a little bigger than going a little smaller to prevent overflow. Or you can use a 9x13 baking dish but the bars will be much thicker and take longer to bake.
In bowl fitted for a stand mixer, cream butter, sugar, and salt until light and fluffy - about 3 minutes. Add eggs one at a time, scraping down the sides of the bowl, as needed. Add extract and mix together. Add flour 1 cup at a time, just until dough is combined.
Measure out 3 cups of dough and spread onto greased baking sheet. Spoon pie filings on top of dough, making sure to alternate spoonfuls of cherry and blueberry so they are mixed throughout the bars. Spread to edges of the dough and smooth out the top.
Drop remaining dough by the spoonfuls over the pie filing. You want to create a speckled look so no need to smooth the top. Bake in preheated oven for 40-50 minutes, or until tops are golden brown. Remove from oven and let cool completely before icing.
Mix icing ingredients together and drizzle over cooled bars. Cut bars and serve. Store leftovers in refrigerator for maximum of 2 days.