Preheat the oven to 325°F.
Generously butter a half sheet pan, approximately 18 x 13 inches, or coat it thoroughly with baking spray.
Set the prepared pan aside.
In a medium bowl, combine the all-purpose flour, baking powder, and kosher salt.
Stir until evenly combined, then set the dry mixture aside.
In a large mixing bowl, beat the softened butter and granulated sugar until light and fluffy, approximately 3 to 4 minutes.
Add the eggs and almond extract to the butter mixture.
Beat until the ingredients are fully combined.
Gradually add the flour mixture to the wet ingredients.
Mix until the dough is well blended and evenly combined.
Transfer the dough to the prepared half sheet pan.
Press the dough evenly into the bottom of the pan.
Use your fingers or a pastry roller to smooth the top of the dough.
Using a pastry brush, lightly brush the milk over the top of the dough. Use only as much milk as needed to lightly coat the surface.
Sprinkle the sliced almonds evenly over the top.
Press the almonds lightly into the dough so they adhere.
Bake for 20 to 22 minutes, or until the bars are set.
Do not allow the bars to brown significantly; they should remain light in color.
Remove the pan from the oven and allow the bars to cool for approximately 5 minutes.
Cut the bars into portions with a sharp knife.
If the bars are too soft to cut cleanly, allow them to cool slightly longer before slicing.
Carefully transfer the bars to a wire rack.
Allow the bars to cool completely.
To prepare the glaze, whisk together the powdered sugar, almond extract, and milk in a medium bowl until smooth.
Adjust the glaze consistency, if needed, by adding a small amount of milk to thin it or additional powdered sugar to thicken it.
Drizzle the glaze evenly over the cooled almond bars.
Allow the glaze to set before serving or storing.